a) if you had the ACW mod loaded, this shouldn't cause any trouble, just restart the launcher and off you go...
b) but if you were Vanilla ready before, when the ACW launcher starts it automatically starts copying core mod files into Empire Total War/data folder from Empire Total War/ACW/Packs. If it crashes during this procedure, it could mess up the file bg_art.pack, which it starts copying first. In this case it only copies a part of the file, therefore placing an incomplete file in the data folder. When you then relaunch it, it will copy the other files, but not this one. So a corrupted file is left there and when you launch the game it will eventually cause a crash. This file contains a lot of the UI backgrounds, flags, etc, so your game might crashe before finishing loading, or when you wanna start a campaign or whenever the game needs some resource from the corrupted file.
How to fix? After crash, run the launcher again. It will finish copying files. Then go to Empire Total War/ACW/Packs and copy bg_art.pack from there toEmpire Total War/data manually. The uncorupted bg_art.pack should have 83MB, the corrupted one is usually smaller... Note: If you do not do this and then exit the launcher with "Vanilla ready" the corrupted file will be copied over the good file and you will get crashes on all subsequent tries. Only a reinstall of ACW will help you then.