This has just started happening to me.
I have 2 waiters unlocked and both were showing up for work quite happily for several days. At my most recent visit to the restaurant, neither waiter showed up.
I tried closing the restaurant, leaving the lot and coming back and re-opening the restaurant, without success. ETA: I tried this multiple times.
I then tried firing both my waiters, leaving the lot, coming back and hiring new waiters. Nothing.
However, when I tried this a second time, one of the two new waiters showed up. The other didn't.
NB: I have no mods or custom content installed.
ETA:
Additional note - while it's been a known issue that sometimes one or more employees don't show up for no apparent reason in other venues (retail shops, vet clinics), restaurants are a more serious case. If none of a particular employee type shows up, the restaurant *cannot function as intended* and there is nothing that you as the owner can do to work around it.
Your only option seems to be to fire all of that employee type, wave goodbye to all the money you sunk into their training, and then hire new staff. Or according to a post on a separate forum, sometimes to delete the restaurant entirely and place a new copy.
ETA 2: Reopened the restaurant on the next sim day. Neither waiter showed up. I've sold it now because it's just not worth the aggravation until this is fixed.
