This is a common issue faced by MySQL administrators generally. I have also faced this issue many times on various server and almost I fixed this issues using one of below given solutions. We hope following solutions will fix your issue also.
Cause 1: MySQL service stopped
Solution : To fix this issue start mysql daemon.
service mysqld start
Cause 2. Not enough space on / ( root ) partition.
Solution: Check and free some space in root partition and restart MySQL service.
# df -h
Cause 3. Permission issue with /tmp directory
Solution: Assign appropriate permissions to your /tmp directory.
# chmod 1777 /tmp