HELP!!! computer stuck in post screen will not boot , what can i do

[GARETHARCHER]

hi i have a problem with my computer , it is a clean pc no programs or anything on it , extremely basic . when i turn it on it is stuck in post screen , it gets up to were it says , found D drive bladi bladi bla , and then stops , nothing works , very ocasionally when bin disconected from everything for a month it will work for a few hours then stop again , what could it be ???? plz help .