Electric shock touching cpu case

Hello,my computer was working well until this problem occured. The problem is that whenever I touch the cpu case and try to connect usb flash it gives me electric shock. I think some uncovered wire is in contact with the case. I didn't try to solve it by myself because I thought I have to get some help on what to do on this before I do anything. Somebody help me please. What shall I do? Thanks in advance.