Did you perform any SolarWinds upgrades prior to the connection breaking? I've noticed something like this in the past and the workaround I found was to just re-create the connection in Alert Central. Copying over all my custom group routing took some time, but after making a fresh connection to the Orion server, alerts began coming in again. You might give this a try to see if that can solve your problem.
The only thing I did before it stopped working was upgrade the licenses with our new renewals.
as of this morning I redeployed the alert central OVF and set it up from scratch for the third time and we just started to get alerts. I'm still confused as to what caused this. Hopefully it doesn't do this next year when we renew or update licenses.
Recreating all the alert central connections was the first thing I tried. Nothing would come in. it was weird.
now just setting up my rules again will be the fun part.