[AccessD] update wouldn't run

I ran into a really weird, and a bit humiliating situation this afternoon. I volunteer at our county fair and someone asked me to help them update some entry numbers in an Access table. It was a simple update query -- make all 20's 21. It was a text field. I created the query, ran it... it wouldn't run. It did nothing. The text field was part of a natural key primary key, but she was able to change the values manually. That query should've run and changed all those values in a second -- fortunately, there were only a few dozen, but I was still... deflated. :( 

Any ideas what might have been the problem? 

Susan H.