A blank page ('white screen of death') is usually caused by a fatal error in the PHP code. Since you said that you updated your theme (although it is not clear whether you personally updated the code, or if the theme was updated by the theme developer), then the problem is most likely in the theme.
You can rule out other problems by changing to one of the default WP themes. If the page is displayed OK, then it's probably the theme code. In that case, you need to contact the theme developer through their support forum to get the fix.
If you can't see anything (including admin-level access), then you need to disable the theme. Do this by using your hosting cPanel's File Manager (or your favorite FTP client) and rename the problem theme folder (in the
wp-content/themes folder). That will cause the theme to default back to one of the WP default theme, and you should be able to log and fix things.
You can fully determine the cause of the error by looking at the error.log file - via the cPanel File Manager (or your FTP client). Look at the last error. It will tell you which file caused the error. Chances are that it is in the newly updated theme folder.
It's slightly possible, in your case, that a plugin might also cause the error. You can temporarily disable all plugins by renaming the
Note that either 'disable' process will make the site look funky (not normal), so you might take a look at the error.log file first to determine the actual problem.