Where did you purchase your NOW TV playback device from?
If you think it's malfunctioned, under the 'Sales of Goods Act' your first port of call is with the vendor.
Welcome to the forum. Is that with one of the catch-up apps (ITV Player, All4, 5?) if so, I think most of us have experienced something similar in the past, All4 seems particularly susceptible on occasion. Unfortunately these are third-party apps so the way they stream is down to the individual providers rather than NOW TV. However usually the problem goes away after a day or two, you could also try rebooting your broadband router to see if it helps.
If you are still having problems in a day or two post back, but it's usually intermittent issues on All4’s end and I usually find it randomly starts behaving itself again. As mentioned no harm in rebooting the router though, it might help.