Pilot light is on. Electricity is going to unit. Won't work after that and don't know why.

Checked thermostat? 24VAC at gas valve?

Here are some manuals: https://www.dropbox.com/s/5i6og7dqk1...nuals.zip?dl=0 Almost all units are about the same. You will need a meter to be able to test the various circuits. You can find a good meter at Home Depot or Lowe's and many other stores. Do not do anything without testing with a meter.