First off, go and sign into your hostings CPanel dashboard.
2. Go to your file manager and click it.
3. Select the website url which has the problem in order to go to its root folders
4. Double Click on wp-content
5. Double Click Themes
6. Find the theme with the problem from the list
7. Right click the theme name and choose "Rename" from the list.
8. Edit the name of the problem theme.
Example: my theme was Eleven 40, I changed it to Eleven 401. Anything will do.
9. Press save.
Now what will happen is because you renamed the theme with the problem Wordpress won't be able to find it, so Wordpress will now revert to one of the "Default" themes. This then gives you the chance to just log into your Wordpress dashboard and select another theme. Job done.
Please Note: Do not re-activate the theme you had the problem with. That theme will need to be deleted and re-installed before using it again. I'm pretty sure it will just give you the same error if you don't delete and then re-install a fresh copy. Also remember to rename the problem theme back to what it originally was. This is required in order to delete it. If its not renamed back to the original theme name then it can't be found in order to be removed / uninstalled. Hope this helped you out.