report works, query doesn’t

In Access 2000 I have a report based on a query. The report works as desired. When I start the query by itself I get “Invalid operation” and an empty Help screen. I also get “Invalid operation” when I try to start a new form I am basing on the same query. I have executed the query by itself in the past, and I do not believe I have made changes in it without executing the query directly before running the report. What might I be doing wrong?