Mongodb used to work well before. Today, it suddenly started to report an error. According to the Internet, the possible reasons are:

1. failed to start when service mongodb restart
2. caused by not shutting down mongodb properly, such as direct kill -9 <pid> causes
3. locked by multiple threads when inserting data [may be too much data, space capacity is insufficient].

My solution:

Delete the mongod.lock file under dbpath, then delete all log files under logpath, and finally execute reboot to restart the server side.