Make sure that the x-box live system isn't going through some type of update that causes the live network not to work for anyone. If it has nothing to do with that than,
make sure your x-box live cable is secured connected between the x-box and your wireless router.
If they are, than you could try un-plugging and re-plugging everything in, or you could try just simply restarting the x-box. I've had the same type of problem before as well and this is what I've had to do.
If you can't find a solution, you could always try contacting Microsoft and see if they can help you with your problem.