Normally, volume groups are automatically activated during system startup. Unless you intentionally deactivate a volume group using vgchange, you will probably not need to reactivate a volume group.
However, LVM does require that a "quorum" of disks in a volume group be available. During normal system operation, LVM needs a quorum of more than half of the disks in a volume group for activation.
If, during run time, a disk fails and causes quorum to be lost, LVM alerts you with a message to the console, but keeps the volume group active.
If there is no other way to make a quorum available, the -q option to the vgchange command will override the quorum check.
vgchange -a y -q n /dev/vg01
You should attempt to return the disabled disks to the volume group as soon as possible. When you return a disk to service that was not online when you originally activated the volume group, use the activation command again to attach the now accessible disks to the volume group.