If you server crashes during a yum operation, your yum metadata can get corrupted causing issues, and you have to rebuild it.
rm -f /var/lib/rpm/__db*
yum clean all
rpmdb: Thread/process 7940/140325995558656 failed: Thread died in Berkeley DB library(Comments)
error: db3 error(-30974) from dbenv->failchk: DB_RUNRECOVERY: Fatal error, run database recovery
error: cannot open Packages index using db3 - (-30974)
error: cannot open Packages database in /var/lib/rpm
Error: rpmdb open failed