What happened next?

I chose something from my WIP folder to work on. It's a short story written on the back of some A4 sheets of paper that had sales figures on the back. So clearly I had got this idea when I was at work and grabbed whatever I could to write it on, probably during my lunch break. But that break must have ended too soon because the story is almost finished. Unfortunately it stops mid sentence. I guess the phone must have rung or something and I never got back to it. As first drafts go it's not bad, but what on earth happened next? I don't know how I meant to finish this story. I've just typed it up, trying to get a feel for it again, hoping that the ending would come to me. sadly it didn't. I'll be going to work tomorrow, maybe it will come to me there.