I had this happen recently due to a logic error in my code. When using modal forms you have to make sure that when the last visible form is closed that you execute the End statment. Example that cause a problem:

You have a "main menu" form with a "reports" button on it. User clicks reports button and the form opens up in modal state (frmReports.show 1, Me)while the main menu form becomes hidden (frmMainMenu.Hide). User clicks the Close button in the control box of the reports form. While there is now no user interface the main menu is still alive and well in memory as it has not been told to shut down.

Have a check through your code and make sure this is not the case. If younhave the control box accessible ensure that you do a trap on it to make sure you can issue an End command (if so desired).