ATCTech pretty much nailed it. But for the official break-down:
ATC-Alert is a precautionary notification to ensure Support Facilities in an OCP network are informed of a possible ATC-Limited or ATC-Zero declaration. ATC-Alerts are normally associated with, but not limited to, non-routine maintenance activities or equipment outages that eliminate backup equipment to critical systems and services. ATC-Alert declarations include (1) the condition that initiated the alert, (2) actions being taken, (3) potential impacts to air traffic, and (4) when an update is expected. 1. ATC-Alert is normally declared by the Impacted Facility. 2. ATC-Alert should be declared whenever the facility is within a single point of failure from being unable to provide published air traffic services in one or more operational segments. d. ATC-Limited is the OCL declared when a combined Tower/TRACON or multi-area Facility is unable to safely provide published air traffic services from one or more Options/Areas while others remain in operation.
ATC-Limited – OCL declared when a combined Tower/TRACON or multi-area facility is unable to safely provide published air traffic services from one or more Options/Areas while others remain in operation.
ATC-Zero – OCL declared when it is determined that the facility is unable to safely provide published air traffic services, or traffic flow management in the case of the ATCSCC.
So depending on the facility, other neighboring facilities might be able to take over from the ATC-Zero facility, in a very limited fashion. So if a center goes down, the neighboring centers and TRACONs take over the airspace per contingency plans. Same if a TRACON goes down, the center will attempt to take over the approach airspace. If a tower goes down, however, it's game over. Nobody can take that airspace, so it is generally treated as an uncontrolled airport, IFR services handled by the overlying TRACON or Center in a very limited one-in, one-out non-radar fashion.
We have seen this very recently with Midway, Las Vegas McCarran, LaGuardia, Salt Lake City airport, and a few others. The airline ramp controllers were still controlling the ramps and associated non-movement areas, but outside of that, the pilots were announcing their intentions on CTAF and doing their thing. At McCarran, for example, the Airport Arrival Rate was reduced to 6 aircraft per hour...so a plane landing every 10 minutes, instead of the usual 40-50+ per hour under normal circumstances.
Probably WAY more than you ever wanted to know, but there you have it. Ha ha!
