Weird blue vertical bar appeared. Now I can't play back the score.

Dear MuseScore people,
I have a weird problem, and I can't figure it out. 🤔

I just opened one of my scores, and I pressed play. It is refusing to play,
and a weird blue vertical bar has appeared in the score. I have no clue
what this is, so I don't know what to search for in the Handbook.

Can anybody tell me what this blue bar represents?

And how do I get out of this mode so that I can go back to normal and play back my score?

Thank you for any help you could offer me! 🙏