In Content Manager, try selecting the Open for Editing filter to see if it's there. If so, revert or submit depending on whether you made changes which have yet to be saved.
If it's not there, try the Routes and Sessions filter and enter your session in the search box. It may show up as faulty, requiring further investigation to determine the cause. If it's not there then it has somehow been deleted from your PC. First try a database rebuild; after that you'd be relying on back-ups.
