Your rig could be offline due to many reasons:
First please check if your rigs are still mining in pool.
If yes then your rigs just have problem connecting with simplemining.net and they will appear as OFFLINE in dashboard.
Most common problems are:
1. Local internet or power proglem - check if your rigs have power and internet connection
2. Rigs can crash/freeze and be unable to work, reboot rig to see if that helps
3. DNS problem - sometimes DNS you are using can cause problems like that.
First try to chang DNS in your router to most stable Cloudflare DNS servers: 1.1.1.1 and 1.0.0.1
After that, please reboot your router and after that reboot also your rigs and see if that fixed your issue.
4. If rig is mining in pool and it is offline in simplemining then also usb drive can be corrupted or broken (try other osb drive)
5. If problem still persist then please check cloudflare anti DDoS service if they have curently any problems
https://www.cloudflarestatus.com/
If they have curently some problems then most common they will fix it within 2 hours.
6. If the problem is still there then you can check if you are not blocked by cloudflare due to suspicious activity from your internet provider.
If nothing above helped, please contact our support.
Also it is good to plug in LCD monitor and reboot rig and see what is going on.

The answer is NO you cannot mix them in ONE rig.
In one rig there needs to be either AMD only or Nvidia only.
AMD has different overclocking schema which is not compatible with NVIDIA.

1) Make sure you are running the latest BIOS! (you can also try older ones if newest isn't working)
2) Set device to boot from USB/pendrive !
3) Disable Sound audio card !
4) Enable 4G encode !
5) Set PCIe support to Auto or GEN2 !
6) Enable onboard graphics to default (PEG - > IDG)
7) Set also: power ON on AC power lost, disable virtualisation
8) Plug your monitor into the ONBOARD graphics if you want to take a look on boot process

TB250-BTC :   use this bios firmware version because other cause problems: B25CF330.BSS
Bios-Chipset-mining mode-Above 6x VGA
Bios-Chipset-PCIO Config- PCI Express Config - GEN 2(or maybe Auto)
Bios-Chipset-PCIO Config-HD Audio -disabled
Bios-Chipset-System Agent configuration - Internal Graphic- IGFX
Bios-Chipset-System Agent configuration - Primary Display - Onboard 
Bios-Chipset-System Agent configuration - VT-D - Enabled
Bios-Chipset-System Agent configuration - Above 4G - Enabled
Bios-Chipset-System Agent configuration - PEX 16_1 - Gen 2(or Auto)
Bios-Boot-Boot option filter - Legacy only
Bios-Boot-Storage-Legacy
Bios-Boot-Option #1 - Your SSD legacy
Bios-Security-Secure boot - disabled

Asrock h110 pro BTC (  installing your GPUs, make sure you have filled all black PCI-E slots first, before plugging your risers into a white slot)
Bios - Advanced - Chipset configuration - Primary Graphic Adapter - ONBOARD
Bios - Advanced - Chipset configuration - Above 4gb - Enabled
Bios - Advanced - Chipset configuration - Pci link speed - GEN 2(7 times)
Bios - Advanced - Chipset configuration - Audio - Disabled
Bios - Advanced - Chipset configuration - Onboard HDMI HD Audio - Disabled
Bios - Advanced - Chipset configuration - Restore AC power loss - Power ON

Asus B250 mining expert  ( this motherboard is a leader by the number of reported issues. Keep that in mind )
Tool - EzFlash - Update bios online
Boot - Fast boot - disabled
Boot - Next boot after AC Power loss - Normal boot
Boot - Secure boot - disabled(legacy)
Advanced - mining mode - Enabled
Advanced - Post slot detection time - 10 sec
Advanced - System agent Config - Primary Display - CPU
Advanced - System agent Config - DMI/OPI conf - GEN 2
Advanced - System agent Config - PEG port conf - GEN 2
Advanced - PCH Config - PCI express config - GEN 2
Advanced - Onboard devices config - HD Audio controller - Disabled
Advanced - APM config - Restore AC Power Loss - Power ON

Z270-P : please plug in pendrive in USB 2.0 (if you have problems with your rig)

If you still have problem booting when pendrive is in blue USB 3.0, please try plugging it into black USB 2.0 also.
Also if you still have problem, try redownloading image and reflashing it again to USB (maybe it get corrupted while downloading).
If that will not help, try using other pendrive - sometimes pendrives just broke.

Most common cases:
1. Too much overclocking or undervolting (or custom bios with bad settings)
Lower your overclocking a lot and check if it will help.
If you have custom bios then check if flashing original will fix the problem.
If you have powerstage level in overclock menu set to 6, try chaning it to 7.

2. Bad riser
Low quality risers are very common in causing many problems.
Change riser or use better ones and see if that will help.
Try checking one by one GPU to see if maybe only one is causing problems.

3. Too weak Power supply can cause that rig will reboot or shutdown.
Also too high temperature in room can cause that power supply will just shut off.
For example some Corsair Power supplies can shut down itself if temperature is bigger than 39°C.

This error is very common and it can be related to
1. No gpu can be found by your system
To check how many cards your system found press CTRL-C while the error is being displayed and type:
lspci | grep VGA
If it only shows the integrated gpu from your cpu then your system can't find your mining GPU. Please check cables, riser, power etc.
2. Internet is very slow or does not work at all and the mining process cannot start.

This message occurs mostly in nvidia GPUs when:
- you have entered too much overclock values. Try lower ones like core: 10 and mem: 10 for testing
- riser or risers are broken
- there is problem with Power supply that makes setup unstable