Internal flash not working because of non compatible flash installed

Solution:
Look at the external flash socket.
With the camera in "normal position", i.e., poiting away from you, you shoul see a very small (I realy mean very small) piece of plastic under the rigth inside of the flash shoe... it is the "external flash sensor". It probably is stuck. Work with the point of a knife to try and release it.
In more detail, if you look carefully at the external flash shoe, the are two metal pieces that will help create some pressure. So the flash holder (shoe) is made of two parts: a very rigid one and the metal blades that create the pressure. Look under the blade located nearest to the shuter button.
If you can locate the plastic piece (black in my camera, but can be white) try to have unstuck.
Worked with my camera.
regards
Mig