I have a forced air heater that is connected to the boiler in my condominium building. I replaced the very old 1970's line voltage thermostat with a new programmable line voltage thermostat (Aube by Honeywell TH106/U). There were only 2 wires to connect. Once I connected it and turned the heat up to activate it, I heard a popping noise from the motor area. Now if I try to turn it on, I hear the fan cycle up for about a second, then cycle down. The thermostat displays "LP", which in the users guide says:

The thermal circuit breaker in the heater is opened. This can happen if the heater is obstructed by furniture or curtain and has overheated, or if the thermal circuit breaker is defective.

I'm wondering if I somehow blew a fuse in the motor area. The only thing that looks like I fuse is the box in the attached picture. Any ideas of what this might be and how to fix it?