Reset

Try whats been advised, then try the FAQ here - http://forums.forza.net/turn10_postst6805_Forza-Motorsport-5---Game-FAQ.aspx.

If you continue to have problems check this thread and try the steps there http://forums.forza.net/turn10_postst6123_LOAD-FAILED-after-update--SOLVED.aspx and if your still having problems post in that thread.

Thanks