Volume won't mount

The volume has an error, thus it will not mount.
There is a variable which control the automatic load of vrepair when attempting to mount a volume thas has an error.
If not load vrepair.nlm select the volume and follow the on screen procedures.

What's Netware version? Try to load from DOS partition of C-drive:
load C:\nwserver\vrepair - something like this. Is is SYS volume or any other volume?

Either something wrong with the volume (corrupt FAT etc), but that should be limited to one volume then. If none mount, you better have a good backup because your disk is gone. Even if DOS still boots, NW is a bit more sensitive than DOS.
Checkin INSTALL (NW3-4) or NWCONFIG (NW5) if you can still see the disk (=driver loaded correctly?), and check if there is a partition table. Possibly NetWare claims there is none, or an inaccessible one. With a little luck an update of your disk driver may cure things then. If the partition is perfectly readable, but still the volumes wont mount, most likely you have some damage to your disk, not enough to damage the first few tracks (=DOS, partition table), but enough damage to make NW refuse to mount the rest.

Jim