Flight Service uses a different set of radios and can relay messages to ATC, but they've been slowly dismantled over the last two decades and wouldn't have the capacity to step in for an outage (nor should they be expected to - that's not their purpose).
Some aircraft have CPDLC for text-based communication, but it's asynchronous and you can't use that as a replacement for voice communication in the terminal environment. Also, many aircraft aren't equipped to receive it.
Pilots are trained on lost comms procedures, so if you're in the air and the radio fails they'd fall back to those regulations. Basically all the aircraft would fly their last assigned clearance, and attempt to deconflict manually with other aircraft over air-to-air radio (similar to flying into an untowered airport).
There are contingencies for radio outages, but yeah, everything going down at once (if that's what's happening here, which I doubt) is bad.