Need more details to answer to you. Following are cases when such problem may occur:
1. You used local /private variables for form object reference. You put 'LINKED' into 'DO FORM' statement, so forms released when variables released.
2. Form is released really because some internal code. Before activating of form, check that it is exist. When it does not exist - run it again.
3. Form was modal. When you hide it (by any way), its modal state finished. Finishing modal state means running of code just after 'DO FORM' for that form. When you have clean-up code there, form is released.