I presume you want to open the contact sheet with the same arrangement you did last time, right? Just select Sort by Arrangement again. IIRC PM saves the arrangements in a small file in the folder, so it does not disappear. If you would like to make it more permanent (for sending to a client, for example), you should rename the files after the arrangement in some sequence using the powerful rename function (usually, when sending the files to the client, I do append the original filename or image number to the new sequential filename, easy to do with variables, to help me with locating the original file if ever the need arises)