The mcat file is located inside a container in your Pictures Folder. The container is called Lightroom Library.lrlibrary. Right-click on it and select Show Package Contents to see it.
If you rename this file to Managed Catalog.Old, your catalog information will download from the cloud to your computer restoring your catalog to its last synced state. Any changes made since you were last successfully synced may be lost.
Example, you might have to launch and perform your recent Add Photos again, depending upon their synced status at the time fo the power outage. Any not-synced develop/metadata settings might have to be done again.