I have an old bryant furnace model # 3946aw036100. The thermestat has no control on the furnace and it just keeps heating the house , the only way to stop it is to turn off the main switch on the furnace. If any one has any ideas ,pleas help

Could be one of a couple different things. Did this just start happening? Id start with the thermostat. Has it ever been replaced or is it old as well? If so, that may just be the culprit. You can find one at your local home improvement center or hardware store for $20.-$50.- on average, depending on the features your looking for.