It's Russian Roulette. Most of the time, just apply power to myServer controller and it should boot fine. Then, do a graceful reboot to ensure it's a clean shutdown / restart.
If it doesn't boot, then you will have to build a second boot drive (are you using the Shelf / NVME or a USB boot media?) using the Allonis "Master Image". After that is all booted, insert the bad USB. Shutdown the new USB myServer instance. You might then be able to move the old myServer db and wwwroot folder et al to the new USB. A myServer restart and you should be good to go.
Allonis can be retained to rebuild the system as well.
Hopefully you also made backups of your myServer system to another storage drive that can be reapplied if the old USB is completely corrupt (not too likely).
After you get that all back working, you should have myServer monitor your UPS so that an Automation Event can monitor the battery level and do a graceful shutdown of your system before it dies.
allonis.com/support/documentation?view=category&id=389
And of course, Allonis sells compatible UPSs to avoid such problems.