won't even turn on at all. replaced thermostat. mom says she didn't mess with it but has military time on it now.

Try connecting the red and white wires together. If that doesn't start the heat, the problem is not at the thermostat. If you don't have 24 volts at the red and white wires, check for a fuse on the control board. If you have voltage, most furnaces follow the sequence of the forced draft blower comes on, then the ignitor, the gas, and a timed delay later, the blower. Post back at which point your furnace fails. Also, let us know if it is a heat pump. If so, most of the above won't work.