I have a briggs and stratton 12.5 it turns over but has no spark I started it the day before and started first key but now it won't start any ideas

Hi,
The key switch may well be bad and grounding the ignition all the time. Not a sure thing, but there will be a wire that goes through the blower housing, often white or yellow, that connects the engine to the frame of the machine. Unplugging it will isolate the engine and if the problem is the switch, this will disconnect the switch from the engine. It would crank, but not turn off in this case. Reconnecting the plug would then do that.
Peace,
Clarke