These are the commands to do an offline repair of a Mongodb database. Bear in mind this process is destructive (Eg if it finds an invalid entry it will be removed).
The repair process
First stop the mongodb service (if running)
service mongodb stop
Now run the repair (Adjust the dbpath as required..)
mongod --dbpath /data/db --repair
Check permissions on the files in /data/db. If they have changed to root / camuser and start Mongo again
chown -R mongodb:mongodb /data/db
service mongodb start
Tail the mongodb logs to confirm it has started correctly
tail -f /var/log/mongodb/mongodb.log