Hmm. Try repatching your ROM (Get the latest version of WFCpatcher!), and disable any cheat codes you have at the moment. (or are you using the AR code method? In that case, verify if your code is still correct)
And please be sure your connection information is still correct. (for instance, press the test connection button on your configuration. If this passes and says the connection is successful, you should be good to go!)
If all else fails, please tell us your decive model (eg.3DS, DS Lite) and your router model. @Snoof
Avatar and signature are from Miss Kobayashi's Dragon Maid. I recommend you to watch it!