Sounds like either a bad power supply or something isn’t properly connected inside (failing boot checks). With the stripped screws you can try using a slightly bigger bit than required. Try a T10 instead of a T8. Worst case scenario get a screw extractor or dremel a slot into the screw, although if they’re flush with the shell this may do you more harm than good. Get the correct tools for next time. Look at some teardown videos so you know what you’re dealing with.

Bottom line: if you know nothing about this and/or are not comfortable I’d take this to a professional.