I tried to update plug-ins and my website went down. I set it to default theme and was told the problem was with the HTC theme and to contact the theme vendor. I am having trouble figuring out what that means and who to contact. Does anyone know what I need to do? I would really like the HTC theme back, as the other ones really don’t work for the way my site is set up and I really liked how it was before.
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Depending which theme with that name you mean here’s a support tab above the preview image.

Just for reference it’s worth considering:
Make sure you are using the latest version of the theme - it seems unlikely that the author would release a version which is not compatible with included plugins
Is it a plugin that came with the theme or a 3rd party one? Which plugin are you trying to update?
Well, I tried to update all my plug-ins so I’m not sure which caused the problem. But I switched to default theme and updated them, so they are all fine except for my GIVE plug-ins that need license keys. When I went to look at the themes and live demo the Hope Charity theme, even that wouldn’t work - it gave me the same fatal error message I got before (when I had the theme and tried to log in) which is:
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 went where you suggested, so thank you! So far I’m only seeing more forums so I will keep looking.