In which case I would suspect one of two possibilities.
1. The location of the User Data Folder has changed so Trainz can no longer locate the assets.
To check this you would need to know where your original user data folder was stored and if that has changed.
In my setup the user data folder is located at C:\Users\pwwar\AppData\Local\N3V Games\trs19\build egytsgh61 ... but this can be moved to anywhere. You may have to do a search for any folders with names beginning with "build"
2. The database has become corrupted.
In which case you will need to run, at the very least, a basic DataBase Repair (DBR), or, at worst, an Extended Database Repair (EDBR)
Also it is worthwhile making sure that your antivirus program has TANE.exe in its real-time file scanning exclusion list.