In the flash shoe on the top of the camera, there is a metal spring plate. Under the plate you will find a white cylandars that goes through the hot shoe (about as thick as a paper clip). When an external flash is pushed onto the shoe, this pin is pushed down and tells the camera there is an external flash in the flash shoe. This disconnects the internal flash. Spillage can cause this pin to stick down. Remove the battery, and then put a little Windex on a clean toothbrush and clean the pinhole. Blow lightly to evaporate. The pin should pop up, but it might take a couple of cleaning attempts.
Another cause could be that sticky liquid spillage has gotten on the pivots for the flash. Once again, cleaned with small amount of Windex on a q-tip may free the pivot. One of the pivots is a screw. Sometimes it is rusted, and needs to have the rust scraped off and then a light coating of grease applied.