Microwaving heats up the oils absorbed by the granola and re-toasts the nuts and grains. I usually place the few spoonfuls I'll be eating that morning into a ramekin and give it about 10 seconds on HIGH. That's just enough to crisp up the granola without scorching. If you're re-crisping a larger batch for several people, spread it out on a plate and microwave in 10 second bursts until it's crispy again.
This is really just a temporary solution. If you let the re-crisped granola sit out for too long, it goes back to its limp and stale state. This is why I find it's best to microwave just what I'm going to eat for breakfast. If you have a toaster oven, re-toasting on the toaster tray for a minute or two would also do the job quite nicely.
(Image: Emma Christensen)