Start-up failure caused by power failure (power failure is very annoying)
Generally, just perform the repair method after power off.
1: Enter the bin directory ./mongod -f /etc/mongod.conf --repair
If the database is indexed and the database is being indexed when the power is turned off, use method two to start
2: mongo failed to start after power failure, enter the bin directory without index startup. / Mongod -f /etc/mongod.conf --noIndexBuildRetry