The problem might be something hooked up when it is not supposed to be hooked up, or there might be a setting in the BIOS that was changed. Or, if something was killed by static electricity. If someone handled the parts without wearing a static grounder or handled the parts while not wearing shoes and standing on a mat or carpet (which would conduct static electricity through the body) and if they touched a part, it could have killed the part. You could also try putting some of the parts that you think might have something wrong with them into another computer (if you can) to see if the problem is with a single part and to see which one it could be.
Just some thoughts...