%%[

  SET @submit = RequestParameter("submit")

  IF @submit == 1 THEN

    SET @user = RequestParameter("username")
    SET @pass = RequestParameter("password")

    SET @lookup = LookupRows(@yourDE, "Username", @user, "Password", @pass

    SET @valid = Rowcount(@lookup)

  ELSE

    SET @valid = 0

  ENDIF


  IF @submit == 1 AND @valid > 0 THEN
]%%

<p>You are now logged in and will be taken to your page</p>

%%[
  ELSEIF @Submit == 1 AND (@valid == 0 OR EMPTY(@valid)) THEN
]%%

  <p>Your Username or Password was incorrect. Please click <a href="%%=RedirectTo(CloudPagesURL({{yourCloudPageID}}"))=%%">here</a> to try again.</p>

%%[
  ELSE
]%%

<form action="%%=RedirectTo(CloudPagesURL("{{yourCloudPageID}}"))=%%">
  UserName:<br>
  <input type="hidden" name="submit" value="1" />
  <input type="text" name="username" value=""><br>
  Password:<br>
  <input type="text" name="password" value=""><br><br>
  <input type="submit" value="Submit">
</form>

%%[
ENDIF
]%%