To make nachos crispy again, I recommend warming them in the oven (as opposed to the microwave). Preheat the oven to 375 degrees F, place the leftover nachos in a baking dish and warm them for about 5 minutes (but watch them closely).  You can also broil the nachos to re-crisp them. Just be very careful not to walk away from the oven when you are broiling leftover nachos. They go from crispy to burnt very quickly.