Without the full specifics about your computer, I can't say much other than it sounds like you've corrupted the database by shutting down your computer too soon after exiting the program.
You will need to do a database repair to resolve this issue.
Start Content Manager.
Choose File
Database Repair then Quick Database repair.
DO NOT TOUCH YOUR COMPUTER WHILE THIS IS RUNNING! The process can take a very long time, even for a quick repair. Let the program do its thing. When it's done, it will say whether there are errors or not.
Exit from Content Manager after the process has completed. Reboot your computer and try again.
In the future, wait at least 5 minutes after exiting from Trainz to be safe.
John