By error I placed a Google analytics tag onto the theme
The error is being caused by the theme.
First remove this from wherever you placed it: <?php ob_start(); ?>
Option 1) remove/undo the errant code from the file you placed it in.
Option 2) replace the file you edited with a fresh version from your backups, or with a fresh version of the file from the original theme source.
If 1 and 2 aren't immediately possible, then you can rename just the html5blank-master theme directory (something like html5blank-master.old or html5blank-master.bak should do) then revisit your site at yoursite.com/wp-admin and log in. The default WordPress theme should take over and let you back, then you can focus on repairing the damaged theme.
[edit] <?php ob_start(); ?> is not actually a fix for the issue with the theme.