Click the Sharing button and ensure that the correct Security groups have been granted access to the Dashboard.  If the issue persists, check the following:

Ensure that the Query exists and returns results:

1. In the dashboard, select the Panel Options icon and select Properties.
2. Note the name of the query selected.
3. In the Query module, search for the query.
4. If the query no longer exists, recreate it and select the newly created query in the dashboard.

Delete the panel and recreate it:
1. Click Customize
2. Unmark the panel that is yielding the error
3. Click the Delete Button
4. Add a new panel with the same setup