how to open up computer?

When does it say going to sleep, before the operating system starts, or after.
If before, try going into the bios setting, different m/c use different methods for that, some press 'Del' some 'F1' some 'esc' should say on first screen, look in there for sleep mode.
If after OS starts, I assume windows, go into safe mode on start 'F8' usually, look for sleep in the desctop properties Check settings on both cases.
If all ok, then, suspect Power supply, or motherboard fault.

SD
November 2011