Hi, if it's an Aqua Magic, there are two possibilities.
One is that some debris is lodged in the area where the slide retracts and it's keeping the water pedal partially depressed. Pulling up on the flush handle will confirm this (and push whatever's in there further into the channel).
The other possibility is that the "ball valve" in the fresh water inlet is so worn that it causes the toilet to try to fill continuously. They can go bad all by their lonesome if pressure is kept continuously on the system.
Getting to it requires removing the toilet. There's a plug under the seat at top left that lets you reach the flange bolt with a long extension, and the other can be reached in the pedal area. It helps to have something like a plastic soda bottle to wedge in there to hold the pedals out of the way while you work on it.
The ball valve can then, if I remember right, be removed without disassembling the toilet.
And, in my opinion, this is why there should always be a shutf-off valve in the line going to the toilet.