How about trying this
1. Create a new form in design view, completely blank doesn't need anything of it. In the code of the OnLoad event or OnCurrent event, do :
Dim lookup as string
lookup = DLookup("[Field name]","table name","[when this field name] = value")
If (IsNull(lookup)) Then
DoCmd.OpenForm "form name to enter details"
DoCmd.Close acForm, "this forms name"
Else
DoCmd.OpenForm "form name when there are details"
DoCmd.Close acForm, "this forms name"
End If
2. Create an AutoExec Macro that loads the form on startup, but the view of that form is HIDDEN! This way the user will never see it.
3. Tell the macro to run on startup of the database (which should then load the blank form but you won't see it) which will then check the table to see if there has been any data entered. If there has been data entered into the form then it will load whatever form you specify, but if there is nothing in the table for the specified record (just use ID 1 or whatever the first record will be) it will load the form to insert the data.
Hopefully this code has helped in some way, if not then put the database on here and i'll take a look at it.
The above code may be very inaccurate, i was unable to check the correct format at the time of writing this, but hopefully it'll give you a rough idea of what to do. Sorry if i've misunderstood what you were aiming to do.