Answer:
Timing belts do not slowly go bad and lead to worsening symptoms. One minute the belt is fine, the next, it snaps and the car won't run. It is possible for the belt to jump a gear tooth if it is not installed properly, but this is apparent as soon as the new timing belt is on and you try to start the engine to test it out.
If the car won't start, it might be the battery or starter motor (it would sound weak)
try these first
If your car still won't start, it might be the timing belt, or the fuel filter (or pump)