before upgrade Drupal, most people will shutdown the site as written in the upgrade instruction.
after or during the upgrade process, you may accidentally faced with a site tells you its is under maintenance and unable to login.

do not panic, and do not modify any source code since there are easier ways available.

there are 2 ways to work out.
for these site that have .htaccess clean url enable. please browser to http://yoursite/user

for these site that don't have .htaccess clean url enabled. please browser to http://yoursite/?=user

for example: as to this site, browse to www.cmsdev.com/usr or www.cmsdev.com/?=user

can you see that? the login page appear, now you can login to continue your work.