"Do not Disturb" won't turn off

[AC Question]

Hello - I've used the 'Do not Disturb' feature on my Nexus 5 many times, but suddenly I can't switch it off - very annoying as I can't hear if anyone calls me, or if I am sent a message. I've tried the 'Volume down' and 'End now' think, but that doesn't work either. Any ideas?
Richard.