If your instructor set up the quiz to open in a new web browser window, Blackboard is trying to open the quiz but cannot since you have a popup blocker on your computer's web browser. Check for popup blockers in your browser.
If this is the case, the next time you click the Quiz link, Blackboard will report that the quiz is in progress. You can ask your instructor to reset your quiz. Then disable your popup blocker in order to take the reset quiz.
If you clicked on your Back button for any reason, Blackboard thinks that you stopped taking the quiz. Then when you go back to the quiz, Blackboard thinks you already took the quiz. If that happens, ask your instructor to reset your quiz so you can try again.