IF you have 24vac across W and C, then you are getting a call for heat from t-stat.
IF temps show t-stat should be satisfied and off, then t-stat may be bad.
Then, with 24vac at W and C, you should have 24vac at the molex connector on board going to gas valve wires also.
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?