la connection wifi fonctionne mal Wifi connection fails

The wifi connection does not work (after working perfectly). I cannot read or send mails and go to the internet. But it works fine with an ethernet cable. Yesterday, I reinstall Yosemite. It allows me to restore a good wifi connection. But after one day I have the same symptom. Wifi connection is no more active or useful (i am connected to the wifi network but nothing happens)