Measure stretching into the margin instead of moving to the next line...

Is it possible that you modified Style -> Edit General Style -> System -> Number of measure per system? It should be on 0 always. If it doesn't solve the problem, click on a barline and press return

Seeing the actual score - as opposed to just a picture of it - would be much more helpful. But generally, this happens only if you've gone out of your way to tell MuseScore to do something that just can't be done. Like if you've told MuseScore it must put 9 measures per line via Style / General / System, or you've selected these measures and reduced the "stretch" down to some crazy negative value via Layout / Add Less Stretch. EIther of these might be settings you made inadvertently while experimenting to see how things worked, not realizing the detrimental effect they would turn out to have later.

So I'd go back and try to undo that - visit Style / General / System and be sure to clear any setting made there, then go to Layout and select Reset Stretch. If that doesn't return things to normal, then post the score (the actual MSCZ file) and someone should be able to see what went wrong.

I've seen this effect before in my scores - however, it seems to be temporary. Adding and removing manual line breaks typically help, or re-loading the file.