What do you do if your nitroglycerin is expired?

Buy more. Ntg is good for 6 months if kept in an air tight amber glass (light resistent) container. If it's opened, it's only good for 6 weeks. After that, it should be discarded and replaced with fresh ntg.
Throw it out. Don't ever use old nitro; it won't work and you'll end up going to the emergency room.