the table is blank, no records, the user needs to enter his password to have 1 record in the tableIt depends of your definition of blank. Do you have an existing record in that table with the field WPassEnt being Null? If yes you can use a Dlookup to check:
If NZ(Dlookup("[WPassEnt]","WPass")="" Then 'assumes the table will only have one record, the one for the current user
'do your Update
Now if you mean the table is blank like in no records yet you code would fail as you cannot update something that is not there yet. You need to use an append instead (INSERT INTO).
For that to check you use dCOunt:
If DCount("*","WPass")=0 Then 'no record exists
'insert new record in table