If you updated to Build 14926, then this is a known issue. The only known solution is a hard reset.
For a detailed explanation of all reset options available for Windows 10 Mobile users, please refer to this thread: http://forums.windowscentral.com/windows-10-mobile-how-guides/438957-what-reset-methods-there-should-i-use.html
are you an insider? this particular bug is hitting insiders on the current redstone 2 build. If you are indeed an insider, the only solution available at present is to hard reset your device or to roll back using Windows Device Recovery Tool.