jump wire make grub wrong & loosen memory make computer fail to start

Today I want to add my former hard disk to my computer in the lab. But after I added it, the mainboard alarmed:di,di,di... So I had to use one disk. But it doesn't work. There was no signal to the display. I thought there must be something wrong with the video card while I knew later a loosen memory caused this. Then I tried two hard disk again and failed again. Thus I gave up that time. However, when I restart the computer it said error 18 and the subject is grub. Suddenly I remembered I jumped wire on the disk. I tried to jumped wire back. OMG! Succeed!
At the time of when the OS failed to start, I was really scared.