I left my birth control in a cold car for a few hours. is it ineffective now should i take plan b?

Excess heat, such as a car in summertime, is a large risk for medications to become ineffective. Cold temperatures are not a problem. Your pills will remain fully effective.

Cold is more likely to extend the time a given pill is at full strength than to make it ineffective. Take the plan b if you wish, but I think it would be a waste of the product and it has side effects than can be unpleasant.