Don34 opened his PC's power supply to clean it. Now it doesn't work. He asked the Answer Line forum for help.
Power supply units (PSUs) aren't really meant to be opened and messed with on the inside. Most likely you accidentally broke something or put it together wrong.
Since you've already done the deed, you might as well try it again to make sure nothing is broken or out-of-place. Pay particular attention to dull-colored, cold solder connections, to see if they're damaged.
But I wouldn't put too much effort into it. Desktop power supplies are cheap, and unless you enjoy tinkering, trying to repair one probably isn't worth the time.
My thanks to Mjd420nova and Rommel for their contributions to the original forum discussion, which also includes several PSU purchasing suggestions.