(1) You should not put any code in your interrupt service routine. You should just set a flag that the interupt has occured, and then let loop( ) start doing what needs to be done.
(2) You probably don't actually need an interrupt at all. Just get loop( ) to check the digital pin, every time it cycles around. The main good reason why you would want to use an interrupt here, is if the incoming signal is very brief and loop takes a long time.