What is the best way to treat burn from a heating pad?

Depends on the: Depth of the burn. Only a trained medical person can tell the depth. If it is a heating pad burn, it is usually superficial, second degree at the most. If it is truly only second degree, topical antibiotic ointment and occulsive dressing for a week should do it. But, more importantly, why did you not feel the burning in the first place? If it is on leg, you might have problems with nerves.

Depends on the exten: Depends on the extent of the burn. You probably need to be seen by your dr.