Welcome to the forum.
Bit obvious, but have you tried new batteries in the remote just in case?
If it seems the remote is definitely not working anymore, you would be eligible for a repair/replacement under warranty, so you could either return to the store you bought it, or if you bought it from the NOW TV website go here
Whilst you are sorting your remote out, if you have an Android or Apple phone/tablet you could download the official Roku app from the relevant App Store and use that to control your NOW TV box.
@Anonymous User when you press the buttons, is there a flashing white light on the box? If there is, it isn't the remote.
Also see this page for more help :
Also if you have an iPhone or android phone, you can download the roku remote app.