You probably need to run vrepair.
There are a number of reasons why a volume won't mount.The top 2 are sys is full or the volume is damaged.
At the server console type load vrepair.
You want to:
1.repair a volume(Sys)
2.purge files
3.write repairs immediatly out to the disk.
Do this until there are no errors.
If that doesn't work ,you'll need to load up another program and we'll need to delete files off the volume so you can mount it.Having a full Sys volume is a very bad thing.
I know hindsight is 20/20,but do you not run backups?
This is a fundemental sysadmin function.
If you value your data and job ,this is a CYA task that is boring and not glamorous,but has got to be done on a regular basis.
If that disk is bad, your data is gone and it is very expensive to recover.($$$$$)
Good luck.