Just in time for Memorial Day Weekend, National Parks Traveler reported yesterday:

The folks at Great Smoky Mountains National Park report that the national reservation system for the park systems’ campgrounds crashed earlier today. As a result, all of Great Smoky’s 10 developed campgrounds will operate on a first-come, first-served basis until the reservation system comes back to life. Those who already hold a valid reservation are OK, according to park officials.
That’s probably the case across the entire national park system.

Kurt includes a link for checking the system’s status today.