If you have a small VPS with less than 512MB of memory, you may struggle to start MySQL.
To get around this, you may have disable InnoDB (if you are not going to use InnoDB this will not cause any problems). You can do this by editing /etc/mysql/my.cnf and adding the line
Once done, you will be able to start MySQL. If you require InnoDB support, you would need to upgrade your VPS to one with a higher memory allocation.