If it's older we have cheap things to try like a new battery and the power off and power button procedure but here there is little to work with.
https://www.cnet.com/forums/discussions/my-computer-wont-boot-keeps-restarting/ for example and others
If you get the MemTest screen but the keyboard is not working I try an old PS/2 keyboard but again the lack of detail has me do wild guessing.
I've been trying to get my computer to boot to a usb for the past couple of days to test out Ubuntu. Whenever I would mess something up in the process, my computer wouldnt even let me get to the BIOS. I changed my boot prio #1 to USB. I was able to get into the "Try ubuntu screen" where it said I was missing a file, I knew how to fix it, but I checked out MemTest. Pressing escape wouldnt let me exit so I just shut off my PC. Turning it on brings this problem.
All of the lights I have in my PC are on, fans are working. There is no display on my monitor, as in the graphics card won't send video. If I try to plug in my mouse or keyboard in any USB ports, their lights do not come on.
I've tried keeping the computer turned off for about 10 hours before turning it on, taking out everything USB, putting in only the USB with Ubuntu, nothing worked. Any thoughts? Not sure if my specs are needed, first post here.

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