If your WordPress site is stuck on:
BRIEFLY UNAVAILABLE FOR SCHEDULED MAINTENANCE. CHECK BACK IN A MINUTE
the solution is FTP into your site and delete (or just rename) the .maintenance file in the root.
CLICK TO EXPAND GRAPHIC:
If problems still occur, rename the PLUGINS folder under WP-CONTENT. If you are now able log in, rename the PLUGINS folder back to PLUGINS but rename each of the folders in the PLUGINS folder one at a time until you figure out which plugin is causing the issue.