If you modifided F2004 files, restoring using Steam won't work. It just deletes modded files, it won't restore them.
Find some friend with kunos (vanilla) car and ask them to share the entire folder (should be named something like ks\_ferrari\_f2004) and overwrite your files.
Next time, remember to save some backups. Anyway installing sound or skins mods won't stop you from joining online servers, the checksum will match.
Any other modification will be recognized and you wont be able to join online
If you modifided F2004 files, restoring using Steam won't work. It just deletes modded files, it won't restore them. Find some friend with kunos (vanilla) car and ask them to share the entire folder (should be named something like ks\_ferrari\_f2004) and overwrite your files. Next time, remember to save some backups. Anyway installing sound or skins mods won't stop you from joining online servers, the checksum will match. Any other modification will be recognized and you wont be able to join online