The others already told you much of what went wrong, just an addition : Do not change settings where you don't know what they do, especially in the Bios or Overclocking software. It can destroy your PC. Fortunately you tried first for your GPU that is quite fail-safe, if you would have done this for the CPU you could have burned it :>
I once also had a PC built by a shop which didn't knew what they were doing (they mounted a CD-drive with only one screw so it deformed the case while being transported, and after returning it the same happened with an hard drive that slammed onto the graphics card and destroyed it) - I returned the whole thing, got my money back and bought somewhere else. Not sure if this is possible for you, but I wouldn't accept being treated like this.