Without changing anything to my Robot nor Machine nor Environment nor Processes in Ochestrator, the processes disappeared from UiPath Assistant. It is an on-premis Orchestrator and is managed by our IT department.
I found that the processes disappeared after the SSL certificate for Orchestrator had been changed.
After speaking to the colleague who changed the certificate I found that he had not done the full procedure.
Performing this solved the problem:
Changing The SSL Certificate For UiPath Orchestrator Website: