My furnace is just over 3 years old and this morning stopped working I changed the filter.. it was pretty dirty:( I know that's bad but the blower will come on but no heat so took panel off noticed that the motor wasn't spinning so got that too work furnace tried to work but still no heat... seems like no gas is coming out tried 3 times and gave up.. I reset and turned off the power.. my question is that what would cause no gas?

Bad ignitor, bad high temp switch, tripped rollout switch, bad gas valve. If the ignitor doesn't come on, and you had a dirty filter, I would check the high temp limit switch.