When there is a power outage at your home, OwnCloud will not operate the same. Hardware needs power. Without power, something will go wrong. Here are the steps to recover your system.

Error message that OwnCloud webpage will give

Internal Server Error

The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log

Reconnect your hard drive

The hard drive mount will have been dropped when the power went out. When the power comes back, assuming the hard drive starts on its own, open your file explorer, and click on your hard drives. Linux Mint will reconnect to the hard drive.

Restart your server

sudo service apache2 restart

Enjoy your server!