Were you using a battery backup or surge protector prior to the power outage.
If not the the power outage may have destroyed a component in your power supply and or mother board.
Are you certain the USB Drive you are trying to boot from is bootable and / or has it been used before.
Plug the USB Drive into a USB Port and restart your computer and see if the bios recognizes the USB Drive.
If so make the first boot from that USB Drive than save changes to bios and exit bios and see what happens.
I'm grumpy because I can be not because I'm old.
The world is what you make of it, if it doesn't fit, you make alterations.
Under certain circumstances, profanity provides a relief denied even to prayer. (Mark Twain)