The "Sleep Button" may be making continuous or intermittant contact. This can be caused by a dirty contact point, especially if you have not used the button for some time.
1. Switch the PC Off.
2. Gently remove the Sleep button by prising it slowly out of its holder. Blow any dust away from the contact area. (NOTE : If you put a thin tissue in front of your mouth before blowing it will prevent moisture from getting on the contacts).
3. Use a magnifying glass to see if the contacts are still dirty. If they are then use a cotton bud and white spirits to clean the contacts. (NOTE : If you don't have white spirits then use a few drops of an alcholic drink with a high alchohol content such as whiskey or vodka)
4. Reassemble the switch and switch on.... hopefully it will now stay on :-)