I have an old drive with exiting LVM data. The drive is plugged in as /dev/sdb.

[root@bt ~]# fdisk -l /dev/sdb

Disk /dev/sdb: 300.1 GB, 300069052416 bytes
255 heads, 63 sectors/track, 36481 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x92fc9607

   Device Boot      Start         End      Blocks   Id  System
  /dev/sdb1   *           1          13      104391   83  Linux
  /dev/sdb2              14       36481   292929210   8e  Linux LVM
[root@bt ~]#

Lets check the status of the Logical Volumes:

[root@bt ~]# lvscan -a
  inactive          '/dev/VolGroup00/LogVol00' [278.78 GiB] inherit
  inactive          '/dev/VolGroup00/LogVol01' [576.00 MiB] inherit
  ACTIVE            '/dev/VolGroup/lv_root' [50.00 GiB] inherit
  ACTIVE            '/dev/VolGroup/lv_home' [315.22 GiB] inherit
  ACTIVE            '/dev/VolGroup/lv_swap' [5.88 GiB] inherit
[root@bt ~]#

So the two volumes on sdb are currently inactive. The easiest way to tell if its just the logical volumes are inactive, or if the entire volume group is inactive, is by checking if the volume group directory exists in /dev.

[root@bt ~]# ls -l /dev/VolGroup*
/dev/VolGroup:
total 0
lrwxrwxrwx 1 root root 7 May  9 17:11 lv_home -> ../dm-2
lrwxrwxrwx 1 root root 7 May  9 17:11 lv_root -> ../dm-0
lrwxrwxrwx 1 root root 7 May  9 17:11 lv_swap -> ../dm-1
[root@bt ~]#

As you can see, you only see the ones that are already marked active. So we know the volume group is what is inactive. Lets go ahead and enable it.

[root@bt ~]# vgchange -a y VolGroup00
  2 logical volume(s) in volume group "VolGroup00" now active
[root@bt ~]#

Had the volume group been active, but the logical volumes not been active, then you would use "lvchange -a y ". This is typically the case in a system recovery. Now you can confirm that it is enabled, by again checking in /dev.

[root@bt ~]# ls -l /dev/VolGroup*
/dev/VolGroup:
total 0
lrwxrwxrwx 1 root root 7 May  9 17:11 lv_home -> ../dm-2
lrwxrwxrwx 1 root root 7 May  9 17:11 lv_root -> ../dm-0
lrwxrwxrwx 1 root root 7 May  9 17:11 lv_swap -> ../dm-1

/dev/VolGroup00:
total 0
lrwxrwxrwx 1 root root 7 May 19 11:42 LogVol00 -> ../dm-3
lrwxrwxrwx 1 root root 7 May 19 11:42 LogVol01 -> ../dm-4
[root@bt ~]#

And subsequently, our Logical Volumes are active now too:

[root@bt ~]# lvscan -a
  ACTIVE            '/dev/VolGroup00/LogVol00' [278.78 GiB] inherit
  ACTIVE            '/dev/VolGroup00/LogVol01' [576.00 MiB] inherit
  ACTIVE            '/dev/VolGroup/lv_root' [50.00 GiB] inherit
  ACTIVE            '/dev/VolGroup/lv_home' [315.22 GiB] inherit
  ACTIVE            '/dev/VolGroup/lv_swap' [5.88 GiB] inherit
[root@bt ~]#