|Power outage events often have power spikes or surges associated with them - those things can cause damage.|
To greatly lessen the chance of a power spike or surge damaging something, you should always have your computer, and everything that plugs into the computer that plugs into AC, whether directly or indirectly via a power adapter, AND the cable that connects the computer to the internet, plugged into something that protects against power surges and spikes, such as a protected power bar or UPS device.
Do you have your computer, etc., plugged into such (a) device(s)??
""This disc is not formatted to play in this region""
Did you quote the exact message?
That doesn't mean the DVD is not formatted - if it's a pre-recorded "factory" DVD that's impossible - it means Vista thinks the region is set wrong, either because of a software problem in Vista, or the DVD drive is damaged.
If it is a pre-recorded "factory" DVD it probably still works fine in other computers and DVD players.
The power outage may have damaged data in the Vista installation.
Try running System File Checker.
When it's finished, reboot the computer, and see if your problem is still there.
The power outage may have damaged the circuits on the DVD drive, and/or the firmware on the DVD drive.
You can go into Device Manager, and in the Properties for the DVD drive model, you can change the region, but only a limited number of times per particular DVD drive, then you can no longer change the region. If you can't change the region to what it should be, the firmware for the drive is damaged - replace the drive.