"Operation System Failed " ?? How to fix this problem?

" Operation System Not Found " ?? How to fix this problem?

Basically i dont know wats going on but i was switching harddrives around to my computer to take some data off of it and when i put it back to the original computer :: it went to a black screen and says Operation System Failed

and when i went into my bios it detected no dvd drive or harddrive.

and i double checked the wireing and its perfect. but it keeps saying the same thing. i know the harddrive is not dead beacuse it reads on my other computer.

i really need help this is my brothers computer and if i mess it up my moms gonna get pretty angry at me. any help would be apprecaite it thank you =D

Do you know what master/slave mean when it comes to hard disks? Also, that it actually matters which plug goes where when you use a 80-wire IDE cable?

You must have changed someting somewhere. Changed jumper positions, changed cable locations, changed BIOS settings.

do you remember how the 2nd machine booted with the new hdd as slave or master on the first ide channel or the 2nd one ???
hope brother has a sense of humor