Either of the two things might have happened here.
You've deleted the recipient schema
You've deleted the underlying table.
If the table has been deleted, you can recover it from the last day's backup if you are maintaining it. In case the instance is Adobe hosted, we do take daily backups which are available for 7 days before being purged.
In case you have deleted the schema, please export the schema from another environment say the Dev instance and import it into the current affected environment. This step has to be followed if you had done customizations to factory recipient schema.
If you are using OOB version, please feel free to hit the Regenerate schema button.
My recipient schema was showing error that @id (primary key) is missing. So I deleted it and tried to import it from the postgres sql table.
But it didn't work. I am new to this tool and so working on local, so no prod/dev environments are there.
For now, I have reinstalled the Adobe campaign and exported the recipient schema to avoid this hassle later.
Also, I have another question here.. In schema I can see some MD5 key being generated, but when I export the schema, it is not in the exported file. So, I am not sure if I import the exported schema then it will work. Could you please guide me on this.