Going to sleep. Please can you identify why my desktop pc keeps shutting down automtically.It

Question

Please can you identify why my desktop pc keeps shutting down automtically.It seems to work fine then it freezes up, i have 2 seperate harddrives that i have interchanged in the pc it runs for a while then shuts down.Some times in the shut down mode if i hold the button in to shut it off when i turn it back on it freezes almost right away.