You need to wait until it is daytime, then level it up again. Pokemon only evolve once they level up, or once an evolution stone is used on them.
If you want to be quicker, just change your ds clock forwards, then change it back afterwards. (If it says changing the clock will change certain aspects of the game e.g. time of day, it just doesn't want you to cheat!)
I hope I helped!
Source: evolving other things like this before.
