500 internal server error

I have just recently installed the Highend theme and after making changes I keep getting 500 internal server error and can’t even get the admin login page up. Have uninstalled and reinstalled several time and same thing keeps happening. At one point I was even trying to turn on the “Under Construction” and kept just saying it was saving for over an hour then when I tried to go to another page got 500 internal server error. very frustrating. Please help