If all of your downloaded (including DLC packs) and custom content has suddenly disappeared, then this indicates that Trainz may have lost the 'link' with the LocalData folder, in which case it will generate a new LocalData folder.

Please try following these steps to ensure that your build is pointing to the correct location; these steps can be used for both Windows and Mac:

Find the path to your current Local Data folder by double clicking on the TANE/TRS19 icon, then click on 'Trainz Settings' on the initial launch screen, then click on the 'install' tab. The path will look something like C:/Users/%username%/AppData/Local/N3V Games/TANE/build xxxxxxxx for TANE or C:\Users\<pc name>\AppData\Local\N3V Games\trs19 for TRS19 (this will look different on Mac)

Shut down Trainz. If on Windows, then ensure that tane.exe or TRS19.exe had ended (check in Task Manager no other tane.exe processes are running)

Navigate to the LocalData location, and find the "build xxxxxxxx" folder(s). 

If there are multiple 'build xxxxxxxxx' folders, then please right click and then click on 'properties' for each one. This will tell you which one is the biggest; when you have worked out which is the biggest make a note of it's name.
Double click on the TANE icon again, and then click on 'Trainz Settintgs' and then go to the install tab. Now click on the '...' button to the right of the LocalData path, and then select the folder that was found in the previous step, then click 'ok' or 'open'.
Now close the Trainz Settings window, then click on 'developer' on the top toolbar and click on 'rebuild database'.
When the database rebuild has completed, please try running Trainz again.

Note: On PC, your C:\Users\%username%\AppData\Local\N3V Games\  folder may be hidden.

To unhide the appdata folder:
Step 1: Open a Windows File Explorer window and go to the “View” tab.
Step 2: Click the “Options” button on the ribbon and then go to the “View” tab on the Folder Options window that opens.
Step 3: Click the box beside the “Show Hidden Files, Folders and Drives” option in the list of advanced settings.
Step 4: Click “Apply” and then “OK” to unhide hidden items, including AppData.

If you used a different location for the LocalData folder before the content disappeared, then please ensure that the correct path for the location you previously chose is being used.

If the content is not re-loaded from the alternate LocalData folder, then it may not be possible to retrieve the missing content.