Hate it when that happens...

First (probably obvious to you already), if you have any older saves, I suggest you try one of them.

If you pretty much had the game won (the endgame can be tedious) and you don't feel like going through another campaign just to play non-Romans, you can unlock the factions manually by editing a text file in your RTW directory. This will allow you to play "unplayable" factions as well.
https://forums.totalwar.org/vb/showthread.php?t=44925