Pc closes down to black screen ?

My pc closes down randomly. The screen goes black and the sound continues, the tower fan keeps working as though the pc is on but the game will be frozen and i loose instant team speak connection. It wont let me turn it off unless i flick the plug socket off then wait a minute, restart and its fine as though nothing happened. this is random. any ideas. Ive changed the monitor recently to see if that was the problem, but its exactly the same. please help. Thanks.