The cable for the front brake on a rim brake has come loose, and I can't get it to squeeze into the barrel adjuster screw properly.
I'd guess that what happened is the other end of the cable slipped (at the anchor bolt on the brake). It probably wasn't tight enough and when the brakes were applied the cable pulled through the anchor bolt. Now the cable seems too loose at the barrel adjuster, but really it is too loose at the brake.
All you probably need to do is to loosen up the anchor bolt so that you can pull the cable back through and then tighten it down again. When you do this inspect the cable to make sure that it isn't starting to fray. If the brakes had been solid and this is a sudden change the cable fraying is one of the more likely reasons that I can think of for the cable suddenly becoming loose.