How Can I Fix This?

This was larks headed onto the zipper like the one on the left. I have no idea how it came undone. I am unable to get it off the zipper now as the gap is WAY too small. Is there anyway I can fix it without taking it all the way off?

I tried spreading the opening but it's want to strong and won't budge at all to get the pull string through.

Thanks in advance!