If the Procom
Model number you referenced is correct, this heater does not have a automatic timing feature to turn the heater On and Off. However, it does have a built-in thermostat control, that turns the heater's burner on, once the room/area drops below the temperature/comfort level you have set it for.Conversely
, it turns the burner Off once it reaches and holds the temperature/comfort level for 5 minutes or more, it turns the burner OFF, again. Keep in mind, because this is a built-in thermostat, it's reading the air temperature closest to the heater.
If the thermostat feature is what you're having a problem with, it may be that the sensor bulb has failed (assuming it's not turning ON & OFF automatically). However, this is a fairly rare failure. Unless, the heater is very old or hasn't been cleaned and serviced annually.
Another situation would be that the Control Knob has been left in the PILOT position, rather then the ON position. If that is the case, the heater will not turn ON, period.
Right now, your Owners Manual is your best source of information on the correct operation of the heater, as well as it's Trouble Shooting Guide.
If you have misplaced the Owners Manual, here's a printable link to it. Just locate the Model Number of your heater, ML300TB and click on the icon to the left of it.http://www.usaprocom.com/manuals.php
Hope this helps you solve your problem.