Allow A Field To Be Edited Once, Then Lock Or Disable

The question is in the title...
How do I allow people to enter data into a field once, but then lock it or disable it from being editted later.
Tried this:
Private Sub SubmittedD_Enter()
If SubmittedD = Null Then
SubmittedD.Locked = False
Else
SubmittedD.Locked = True
End If
End Sub
but the If statement isn't working... it just does the Else statement regardless of wether SubmittedD is null or not.

Thank you.