Since the last reboot of my server I can longer access mongo at all. When I attempt to run the mongodb service I see the following error logged in /var/log/mongodb/mongod.log:

{
  "t": {"$date":"2022-08-16T12:45:18.536-04:00"},
  "s": "F",
  "c": "CONTROL",
  "id": 20573,
  "ctx": "initandlisten",
  "msg": "Wrong mongod version",
  "attr": {
    "error": "UPGRADE PROBLEM: Found an invalid featureCompatibilityVersion document (ERROR: Location4926900: Invalid featureCompatibilityVersion document in admin.system.version: { _id: \"featureCompatibilityVersion\", version: \"4.4\" }. See https://docs.mongodb.com/master/release-notes/5.0-compatibility/#feature-compatibility. :: caused by :: Invalid feature compatibility version value, expected '5.0' or '5.3' or '6.0. See https://docs.mongodb.com/master/release-notes/5.0-compatibility/#feature-compatibility.). If the current featureCompatibilityVersion is below 5.0, see the documentation on upgrading at https://docs.mongodb.com/master/release-notes/5.0/#upgrade-procedures."
  }
}

The instructions for upgrading at the provided link require me to run an adminCommand from the mongo shell, which I cannot do because I cannot access it at all.
The mongo shell is gone:

$ mongo
-bash: mongo: command not found

And the mongosh shell doesn’t work:

$ mongosh
Current Mongosh Log ID: [redacted]
Connecting to:	        mongodb://127.0.0.1:27017/?directConnection=true&serverSelectionTimeoutMS=2000&appName=mongosh+1.5.4
MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017

My lock file is empty, but I attempted to repair mongod anyway to no avail.

$ ls -l /var/lib/mongodb/mongod.lock
-rw------- 1 mongodb daemon 0 Aug 16 12:45 /var/lib/mongodb/mongod.lock

I don’t really care about recovering my databases, just getting mongo back into a working state. However, I don’t even know how I can wipe my databases without being able to start the mongo service at all. How can I get mongo running again?

No, it immediately terminates with the fatal featureCompatibilityVersion error whenever I attempt to start it.

I haven’t attempted to perform an update before, so I’m assuming my databases still have FCV set to 4.4. Is there a way I can check?

Thanks! This worked like a charm. Since I’m on Arch Linux I just needed to install mongodb50-bin from the AUR to downgrade, ran the adminCommand, then installed mongodb-bin again to upgrade my binary back to 6.0.0.

Is it good practice to update the FCV after each major release of mongo? Is there a reason it doesn’t happen automatically?

Good to know in case I run into similar issues in the future.