Is it just the Start button which is missing, or has the whole taskbar (at the foot of your screen) disappeared as well?
If it's the latter, try placing your cursor at the very foot of the screen, to see if it changes to a double-ended arrow. If it does, hold the left mouse button down while dragging the top of the taskbar up to its normal position. (To prevent the problem reoccurring, right-click on the taskbar and then on 'Lock taskbar').
Other possibilities are here:
http://ask-leo.com/my...ton_what_do_i_do.html
Chris