I have a Macbook pro late 2011. It has always been connected to the same wifi and never had any problems. I updated to Yosemite and now my wifi goes off all the time. Most of the time when I the in the password for the wifi it simply says invalid password. It is the correct password.
I have followed lots of articles about this problem with Yosemite and tried all the suggested trouble shooting tips I can find. I even phoneed up Apple care and they just went through the same trouble shooting techniques which sometimes works by chance and then the wifi disconnects again.