If the blower runs, it proves power at the air handler. If warm air flows out of the registers, the aquastat could be out of adjustment or the time delay is malfunctioning. If the blower does not run and there is power to the air handler, a control has burned out or the blower motor needs checking. Do not put fingers or hands in the blower. Aside from sharp edges, it could start up unexpectedly and cause a serious injury.
Many air handlers have a safety switch that shuts off power if the door is removed.
Check that the door is correctly installed. If you have a programmable thermostat, check that it has fresh batteries. If equipped with central air, or the system is zoned, make certain the selector switch is on heat. If you are familiar with the fuse or circuit breaker panel, see if the fuse is burned or the breaker is tripped. If you can access the burner, check the red button on the protecto relay.