Is there a log on the file system?

First make sure the disk group is imported with:
vxdg list

If not, import with:
vxdg import disk_group

If that fails, try:
vxdg -Cf import disk_group # assuming this is not a cluster


Then start the volume(s):
vxvol -g disk_group startall

If that fails, try:
vxvol -f -g disk_group startall


fsck the file system:
fsck -F vxfs /dev/vx/rdsk/disk_group/volume

If that fails, try:
fsck -F vxfs -o full /dev/vx/rdsk/disk_group/volume

Mount it up:
mount -F vxfs /dev/vx/rdsk/disk_group/volume /mount/point

Be sure to replace disk_group and volume with your actual info.

Good luck.

If an actual disk failed, and was not fault tolerant, it will be necessary to rebuilt the volume (and maybe the disk group) and restore the data from backup. If it is RAID 0 ir 5 and has a log you should be able to get it back easily.