Canon rebel xti flash pops up but says it was obstructed

I have a Canon Rebel XTi and it has worked perfectly fine until today. Whenever I try to use a setting that requires a flash (such as portrait), the flash pops up automatically as it is supposed to, but then I hear the three clicking sounds and the Error5 message pops up saying that the mechanism has been obstructed. I don't understand, why would I get this message when the flash pops up fine?