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It's jammed with something. Most disposals have a fitting for an alan wrench underneath to manually jar it loose after you have felt for it (turned off of course!). Another way is to use a wooden broom handle. Place it in the disposal wedged against a blade and firmly push clockwise..
Most often, when it's clogged from disposal activity (carrot shavings, etc.), the U-shaped trap is clogged. Screw off the plastic attachment fittings and you will remove the trap. Empty it. That's typically your problem.
Some common reasons that do not necessitate replacing something are a bad igniter (gas) or heating element (electric). Pilot lights can sometimes blow out. If it's leaking, then more than likely you will need to replace it.
Chances are good either your shutoffs are bad or your faucet. Both are inexpensive to replace.
Do you have kids? Most likely something is obstructing the flow in the toilet itself. Cheap to fix.