Laptop dont start operating system not found

so my fiance lap randomly stop working a couple of days ago, when she try to start it only said "operating sistem no found" i though it was something wrong with the S.O., so i only reinstall windows and it works, not for a while, i install a couple apps that she use and the other day the same thing, crash and "operating sistem no found" them i though it was something with the harddrive, i was going to reinstall windows, just to recover some info that she said and then test other hard drive, when i try to reinstall the windows intaller does not read the hard drive, not even the bios, so i test the hard drive in a notebook and the hard drive works! i test the hard drive of the notebook in the lap and same thing, doesnt read it, after a couple of hours trying to find something in google and messing with the bios, i give up, with all hope lost i turn on the lap and it read the hard drive, why? i really dont know, when i start windows said that needed to repair, so i wait and nothing, said that windows could not repair the poop was wrong with it, so i reinstall windows and do everthing again, and next day the same thing, operating sistem no found, but the hard drive appear on the bios, just doesnt read it
and i cant understand why
so if someone has an idea what is wrong with it i really appreciate

btw the lap is a "emachines d525" something like that, i didnt know the brand until know but anyway, and is like 3 years old but she barely use it

and sorry for the bad grammar, not a native english speaker