Computer will not boot after replacing PS2 mouse

My son replaced a PS2 mouse when computer was still running. Mouse will not work after that. When he reboots the computer the screen remains blank and HDD light illuminates continuously. There are no beep sounds. I suspect it is problem with mouse port. I tried cleaning up mouse port with no success. I even tried a USB mouse but failed. This problem is new to me. Can someone help me out?