If not hold the power button on front of console for about 10 seconds ( until it beeps and shuts down ) once the console is off, unplug power lead from back of console, leave for 2-3 minutes then plug back in, turn back on.
If this doesn't solve it,
Have you called you broadband provider to see if everything is ok?
What speeds do you get?
Connected wired or wireless?
Could also give Xbox Support a call to see if they can help.
Good luck and let us know how you get on.
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