Check your catalog folder and delete any files ending in .lrcat.lock and .lrcat-journal then restart Lightroom.
If you're selecting a LR catalog backup in a folder that doesn't show .lrcat.lock and .lrcat-journal files you have a different issue.
Try removing your LR Preferences file, which may have been corrupted by the the power failure:
If the issue persists try uninstalling LR5.5, delete the Preferences file as described above, and then reinstall LR5.6:
Thank you so much trshaner.
The problem did indeed persist and although I was holding my breath, the ol' uninstall/reinstall trick appears to have worked a treat.
Again, thank you.
Glad that fixed the issue. You may also want to run CHKDSK on each drive in your system:
It's always a good idea to run CHKDSK when a power-failure occurs with the system on or in standby mode. I shut-down my system every night for this very reason. If your AC mains service is prone to frequent outages consider installing a battery-backup UPS.