total mobile noobie

Hi everyone! I know I am very...very daft lol. I just bought an xda lls second hand, and I went to use it today, my alarm went off for work so I hit the dismiss button, then entered my password to get in to text someone, but now I'm feeling like a total idiot. How do I get the thing back to the password screen? Don't want my kids just picking it up and seeing everything that's on there.