not enough information to provide any more specific advice as yet. diagnosing the problem will be incremental, starting with the most basic. knocking loose cables is pretty common and easiest to find and fix. do you get any post beeps that would indicate an error msg? those usually occur if any of the key components are missing, loose, or malfunctioning (CPU, RAM, HDD, etc). Or does it just sort of power up, shut down, and repeat?