Disc Mount problem

I have a hard drive that is partitioned into 3 parts, one where i have been doing my time machine backups, one formatted for the mac and one part with dos format to be available to a pc. Yesterday when i wanted to run a backup, the particular section didn't show. I opened disc utility and did a disk verify and repair and all seemed fine, yet in the bottom it says 'not mounted'. The other 2 partitions are still working fine. How do i fix this problem?