Private Sub cmd_login_Click()

If Trim(txt_username.Value & vbNullString) = vbNullString Then
    MsgBox prompt:="Morate unijeti korisnika", buttons:=vbInformation, title:="Upisi korisnika"
    txt_username.SetFocus
    Exit Sub
  End If
 
  If Trim(txt_password.Value & vbNullString) = vbNullString Then
    MsgBox prompt:="Morate unijeti zaporku", buttons:=vbInformation, title:="Upisi zaporku"
    txt_password.SetFocus
    Exit Sub
  End If

ovdje ide kod koji provjerava u bazi dali korisnik odgovara zaporki
trebao bi imati bazu podataka vjerojatno:
userID
korisnik
zaporka

pa nekakvim SQL upitom iscitati to
strSQL = "SELECT * FROM tbl_login WHERE korisnik= """ & txt_username.Value & """ AND zaporka = """ & txt_password.Value & """"

pa bi trebalo ici kao neki k....palac

Set db = CurrentDb
Set rst = db.OpenRecordset(strSQL)
If rst.EOF Then
    MsgBox prompt:="netocan unos", buttons:=vbCritical, title:="Login Error"
    txt_username.SetFocus
  Else
    MsgBox prompt:="Pozdrav, " & rst.Fields(0).Value & ".", buttons:=vbOKOnly, title:="Login Successful"
    DoCmd.Close acForm, "frm_login", acSaveYes
  End If
 
 Set db = Nothing
 Set rst = Nothing
end sub