For unknown reasons, I suddenly can't 'open' (respond to) gmail -- something about pop-up blockers. How can I revert to the way it was 5 minutes ago?

On an email chain that started about 10 minutes ago, I suddenly can't respond, getting a message that a pop-up blocker of some sort prevents it. Plus the message opens up on a different page with it as the only tab. I didn't make any changes, as far as I know. How can I get gmail to return to the way it was 10 minutes ago?