I'll guess phpbb 2.0.9 or 2.0.10
Then ill guess you skipped the note to .9 & .10 users :p
Then ill guess you skipped the link to the support site :p
Then ill assume you didn't search this topic, atleast a few pages back :p
Doing any of those would have possibly fixed ya right up. Here ya go.
( i actually have that link memorized from pasting it so much... lol )
If you are not on .9 or .10 (which this problem has only happended one time to a user not on those) then ensure register_globals are on for your board.