This is a common failing with Canon pop-up flashes. I've had four Canon's with built-in flashes (two film, two digital) and this has happened to all four at one time or another.
What has probably happened is this: On the camera hotshoe (for an external flash) there are two little pin switches that sense when a flash is installed and prevent the built-in flash from popping up. These switches have a tendency to stick causing the camera to think there's a flash installed on the hotshoe and the pop-up won't pop...
There are two solutions... You can carefully remove the little plate that covers the screws on the hotshoe and attempt to pull the switch pins up, but you have to be very careful as they are extremely small and easy to damage. There's a thread on here somewhere about this, but I can't find it. The preferred solution (at least in my opinion) is to forget about the built-in flash, buy a decent external flash and use it instead. A hotshoe flash is much more versatile and useful than the built-in. I rarely go anywhere without mine...