Try this:
If Me.txt_PasswordInput <> Me!frm_LoginSubform.Form!txt_Password Then MsgBox "User/Password Incorrect!", vbCritical, "Error!"
If Me.txt_PasswordInput = Me!frm_LoginSubform.Form!txt_Password Then DoCmd.OpenForm stDocName, , , stLinkCriteria
If Me.txt_PasswordInput =...