Assuming this is a closed combustion device, they almost all do a post-purge after a fail to fire event. Could be 5-minutes or more to ensure there's no gas buildup before it tries again.
If it starts up, lights the burner, then turns off before it heats the water to temp, it could be the flame sensor is not working, or it could be the circuit is bad on the motherboard. Check the connections for corrosion and any loose or broken connections. A less likely thing would be if the aquastat isn't giving reliable readings, and thinks the water actually then was up to temp. Your user manual is the best source of troubleshooting info, and the installation manual usually has more complete info, if you are are mechanically inclined. There may be a test procedure for the heat sensor.