Slide got stuck on clicking on previous button in storyline in classic player

It's hard to know what could fix it since you did not share your file. But my first suggestion would be on the slide that is stuck, look at the previous and next button triggers and make sure they aren't set to jump to slide - THIS slide, and that they are in fact set to jump to previous slide and jump to next slide. If that's not the issue, I would also check to make sure you do not have triggers to disable those buttons on the slide. If so, you also need triggers to change them to normal. 

If it's neither of those things, you'll want to share your file here or with support@articulate.com so someone can better troubleshoot.