Happened to me many mant times it is because ya knocked ya xbox and the mod chip have come lose just take ya xbox apart and reconect it....

Switch it back on every thing wil be fine. If ya did not install the mod chip in the first place could be a prob . But then in saying that take a look on good old goo**e and ya will find out how to do it Any touble let me know and i will tell ya were ya can go for help in installing it