1) Before assuming the sky is falling, check to make sure that your application is fully up-to-date. Quark never leaves a software at version x.0. There are always .x patches that may resolve your issue. Both versions 6 and 7 have had at least 3 updates each. Updates can be downloaded for free from http://www.quark.com
2) If QuarkXPress presents an error message, search for that message in Google. You are likely to find others that encountered the same error and found a solution.
If nothing is found in a search engine, post the error message here. It is difficult for others to help if all you tell them is 'it won't open' or 'it doesn't work'. In more recent versions of QuarkXPress you can look at the crash log for details.