Website down. Unable to fix theme

Hi. I have two websites running Highend Theme (www.dunev.com and http://www.michaeldunev.com)
The sites have been going offline with some regularity and, when I checked with the server, they told me the problem is with the theme. In one site (michaeldunev.com) they changed the theme, allowing me to access the dashboard. But I want to use the Highend theme I have purchased from you.

The other site (www.dunev.com) still has the Highend theme activated but I cannot make any corrections on the dashboard. Any effort to update plugins, access a page or whatever sends me to a Server Error 500 page.

Very frustrating. Can you help?

Thank you