A "showstopper" is usually a bug that is so severe that it would prevent the application from going into Production. What will be a showstopper bug in your application / system / project will be dependent on the context of what you are testing, the impact of the bug on your customers / users, and the definitions used within your project.
If it stops you from testing it's not necessarily a showstopper - if it's a website, are you testing an "About" page or a payments page? One of these may be a showstopper, one is probably not.
Is it stopping you from all testing? What about tests that you don't have test cases for? What about doing the workaround so that further testing can take place on down-line parts of the system?