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.
Best Answer
On my MacBook Pro I sometimes had similar difficulties. It always turned out that the cause were weak signals or interferences. Please try the following measures:
You can analyse your WiFi environment pressing alt and your WiFi icon in the MenuBar. You will get some info like this. If you hover the mouse over other network names you also get information about those.
This article, for example, has more info on the issue.