This is common problem with Nextcloud, and in most cases upgrade of Nextcloud or some app did not finished.
To turn of maintenance mode in nextcloud and continue upgrade, fallow this simple steps.
STEP 1:
Go to cPanel > File manager > public_html > config > and edit file config.php
STEP 2:
Chen you opened file config.php, fine line where it say: maintenance and change from true to false
That is all, go to you nextcloud and refresh it.