Did you alter your games time?
Regardless of this the simple solution to fixing your problem is to wait another day.
If you want to know how this happened it could be that you asked to transfer another Pokemon/ Item before 24 hours of when you transferred the last. -My last answer
Some games have bad effects when time is altered on your DS, whether it was your fault or not. Some effects could be the game disabling time-based events like the item transfer, or exhibiting strange glitchy effects when trying to access these.
PS: It could be a glitch, and it looks like others are having the same problem hence why I just copied my last answer.