Heater stopped working and running on 2 cylinders

Now we can post a list of the possibilities if that’s what you’re looking for... However I don’t think that will be of much help.

What is your most educated guess please Thanks

Failed head gasket will normally produce thick white smoke (steam) from tali pipe - along with engine misfiring - low radiator coolant level and engine overheating. There are test that will reveal a blown head gasket - however that needs to be performed by a pro. There are more possibilities but we will stick with what the person told you that actually looked at the vehicle was thinking - it is logical. Good luck with it?

Do you have any idea of how much a blown head gasket would cost And will it make my heater start working again when I get it fixed Thanks so much for your help

I can’t quote you an estimate - that will have to be done by whomever is going to perform the repair. Low coolant level will cause cold *heater* output.