Adjust the hotshoe. Loosen it off and check the contact in the middle, as is sometimes stays depressed/gets caught when you remove the external flash. The camera thinks it has a flash attached and prevents the pop-up flash from coming up.

Hi guys, I've just repaired one that had a damaged clip, with enough patience you can re-use the old clip even with both ends broken off (I found this easier than trying to do it with one end still on). If you can hold it in place upright with a pair of tweezers and then plug the LCD in, you can get the clip to stay down. I put some tape over it for good measure and sealed it up straight away.