My xbox 360 wont hook up to live?

alright, so one day my 360 decided not to hook up to live, it actually signed out mid way through a movie on netflix. i tried running the test on it to see if i could reconnect but it wouldnt and the help it offered said my MTU setting wasnt high enough and that i should try connecting straight to my modem but it is already straight to it via ethernet cord... not really big on how it all works but i have an idea about computers and stuff so if you can tell me what i need to do thatd be great thanks.