There are a couple of possibilities.
Are you a Trainz Plus subscriber?
If you're subscription hasn't been renewed, then DLC routes and other content will be disabled and removed until you renew your subscription.
If not, then there's something else going on with your data.
Your Trainz install crashed and now your route and session are left open for editing. To repair this, go into Content Manager and search for your route name. If it shows Open for editing, select and right-click on it and choose Submit.
You will want to check your session here as well.
If the route is faulty, then there's something else going on.
Right-click on the route and choose View errors and warnings. Sometimes, doing this will verify the data and your route will be okay and appear again.
If all else fails, run a database repair.
Click on the Developer menu located at the top of the Launcher and Content Manager.
Choose Rebuild Database
This will run for a time and when it's finished, start Trainz and see if your route and session have appeared.
