Tried to update WordPress and now I can't log in.. FATAL ERROR

I tried to update some plug-ins and my whole site got messed up. I was receiving a 500 internal server error which happened to me a few weeks ago and I had restore our website and it was fine. But this time when they did that and I tried to log in, it isn’t fixed. I am getting this:

Fatal error: Call to undefined function ot_register_meta_box() in /hermes/bosnaweb08a/b2591/dom.margyb/public_html/azmamorg/wp-content/themes/HCT15/includes/tgn-meta-boxes.php on line 815

I am getting my site set to default theme because I NEED to be able to log in to the back end, but afterward I am restoring it to this theme even though it is broken so that our users can still access it easily. So once it is back to Hope Theme Charity… I need to know how to fix this ???

Also, please note that while I do understand some computer stuff, I am not very good, as you can tell by my referring to it as “computer stuff” - so please feel free to explain things in the simplest way possible!

In case of interest:

Thanks for your help :slight_smile:
any other suggestions please