Sianny

Hi all! How is everyone? I need some help please!!! I have a Yamaha YBR 125 55 plate. I went out on her tonight and everything was fine until on the return journey home, i tried to put her into fourth gear and she just wasn't clicking in. So i reduced her speed and put her into second gear fine and then back up into third but when i tried again to put her in fourth it wouldn't go in. I have managed to get home safely and she is going down gears fine and into neutral but will not click into fourth. Can any one help to shed some light on this problem and how i may go about fixing it please. Many Thanks Sianny