Huge probem can't install windows xp as no disc!

ive just put a new power pack on my acer pc and when i turn it on it says operating system not found, i dont have a disc to install windows xp because my pc already came with windows xp on it. can anybody please help me as i need to get on my pc asap and i just cant seem to do it! Any help would be greatly appreciated.
can you get into the bios?? if so set bios back to default , save and exit, does it boot now??

what is the model and model number of pc??

laptop of desktop??

was the power pack an exact replacement from an original??

did you see if it was compatible with your pc and xp??