Have you tried rebooting the box by turning the power off and on with either the switch on the back of the box or at the plug, this has worked for some people, or pressing the 'TV' and '0' buttons for 10 seconds and it should now repair the remote.
If that doesn't work you could try a factory reset which works for me when this happened with my remote.
It's only a partial system reset and not a complete factory reset. You are given 2 options,
Keep Recordings
Format Disk
Choose Keep Recordings, that means you won't lose your recordings and any planned recordings or series links you've set up, you will have to sign into your apps such as iPlayer, Prime, Netflix and ITVX again, also Match frame rate and Standby power consumption will go back to their default settings if you have changed them.
