Is the site mission-critical or just a hobby site?
If it's a year out of date.....you have serious catching up on Joomla core updates to do.
Following on from @john-doe's comments, suggest you try the myjoomla.com service; it audits your site for security and other issues. First audit is free, but it is a subscription service, and may be well worth your time and money. It explains most problems is easy to understand language, and takes you through steps to resolve the problems (in many cases, just click a button to fix them)
It will at least advise if there's serious issues with the site in terms of it being compromised (or suspected of such).
This is a lesson learned; make sure Joomla is up to date including third-party extensions. It doesn't matter if the site is a hobby site, or your bread-and-butter.
I can see on the largely white screen just the login form . Front end is no better, with completely white screen.
I think the first thing to do is turn on the error reporting - in configuration.php file (open in a text editor) and turn on the option there. Also, in your hosting account admin panel (not joomla), there should be an error reporting option there, or you can add it to the php.ini file. (You may want to refer to your hosting's support - either contact them or refer to their knowledge-base, forums or FAQ resources, whichever they have available) for info on how to turn on PHP error reporting).
Also it would help us to help you, if you can run the Forum Post Assistant - download it from the link inthe read box, top of this page. viewtopic.php?f=806&t=969442
. It's not a Joomla extension, it is a single file script, upload it to the joomla folder, and run it from your browser. Generate the report, and post ther esults to a reply on the forum here.
If the site is hacked (not saying yours is), there is only one solution - a complete deletion and restore of the site. viewtopic.php?f=714&t=946026