It happened to me once and it almost drove me crazy.
I was upgrading some plugins via the automatic plugin upgrade feature and something wrong happened to my internet connection and when I tried to open my site, I got an error message says” Maintenance Mode”
I did everything I could to solve this problem but no good, and I was left clueless thinking that my site was gone.
What caused the Maintenance Mode Problem?
After doing a lot of online search I discovered that while doing automatic upgrades WordPress creates a .maintenace file that get disabled after the process is successful, however, when things go wrong during this process, the file isn’t removed and it prevents you from accessing your site.
Going to your site files using ftp and delete the .maintenance file found in the WordPress root/base folder and everything should be back to normal.
Hope you enjoy that.