Where to start looking....

What voltage do you have at the battery? Can you jump it to get it started? If so, do it and measure the voltage at the battery again. If battery measures 13.5 volts or more with the engine running, I would suspect that your battery has had it. Use a regular charger (10 amps or more) and charge for at least four hours. Disconnect from charger, let sit for an hour and measure its voltage again. If it is less than 12.5 volts, take it to a parts shop and have them load test it.