Recently I purchased an external flash, used it for a few days. Then I tried using the built in flash. Oops, it wont pop up at all. Not in auto mode, not by pressing the flash button. So I searched on internet and found out that the flash gets stuck and you have to pull it out (BS). Dont do it!!! Some even say you should use WD-40 I wouldn’t, never.
Problem was when you use the external flash, the contacts in the flash holder (slider) become loose and don’t go back up to the original position to let the camera know your external flash has been detached. Until the camera thinks your external flash is connected, it won’t pop up your internal flash.
The solutions is pretty simple. With your hand nails or something soft, just push lightly the side slider bars of the flash holder (just a bit) and release. The two contacts should be touching up the silver body. Once you do that, the camera knows your flash has been taken off, and it lets you use the internal flash. I was actually trying to use external flash as slave when I panicked to see the flash does not popup anymore.
Sorry for the quality. Pictures have been taken with Galaxy Nexus.
Disclaimer: Use at your own risk Never use WD-40 and never try to pull the flash out