During an upgrade, WordPress can put the website into “Maintenance mode”. This generally displays a simple message when you browse to the site saying the site is currently down for maintenance and to come back soon.
Unfortunately, things can sometimes go wrong (hopefully rarely) and when you go to the home page, even as an administrator, the maintenance page is still displayed.
To remove this maintenance page, you need actually log onto the machine hosting the website. Inside the root folder for your website there will be a “.maintenance” file, something similar to the following:-
Simply delete this folder, if on linux, this can be done as follows:-
cd <name of wordpress site root folder> rm -f .maintenance
You should now be able to access your website again.