I am not too familiar with linux, but we had a power outage yesterday and a volume group is now missing.
Is there any way to get it back vgscan doesn't show it?
You need to try 'pvscan' first, to check all your physical devices for volume groups data.
Other than that I can suggest you to make backups of your data on the disks with 'dd' command so you can restore them later if needed. Corrupted LVM volume group restore can be hard and often you can find yourself worsening that with few fault commands. So it's a good idea to create the backups first.
If your VG consists critical data, you didn't have a backup and didn't feel yourself familiar with investigating LVM issues — hire an experienced admin to do that instead of you.
You can take a look here for article about LVM configuration and recovery, it may help you to understand at least basics.