If you have a table in a forms data environment and then open that table again exclusively, the form loses the table. The alias assigned in the DE is no longer valid because it was used exclusively somewhere else. The reference or memory variable to the name still exists, but it cant be accessed because it has been closed. VFP couldnt destroy the reference, because all the bound controls would blow-up.

You will have to add the table back to your forms DE programatically, or open your DE again.

if my memory serves:
thisform.dataenvironment.closetables()
thisform.dataenvironment.opentables()