Hi Holliw,
Do you only use the 360 box or do you have freeview connected by an aerial to the TV, or a blu-ray, dvd player, or another external device connected so we can definitely say it's looking like the 360, even something like prime or netflix connected by ethernet or wifi to your TV.
If have been able to check any of these and it's only the 360 that is the problem then I would try a factory reset of the 360 selecting the option to keep your recordings, or try changing the hdmi cable and connector on the TV.
I would also double check the picture mode on the TV hasn't been changed by accident, you can set them for all inputs or just individual inputs.