When the administrator of a wifi network changes the network password to something new, previously associated devices that have the old password saved will no longer be able to connect until they re-enter their wifi passwords.
Unfortunately OSX handles this situation in quite an unpredictable way:
Whatever happens seems to have to do with the network security protocol in use and also differ between vendors. Sometimes it will just fail silently, sometimes it pops up a "connection timed out" alert box, sometimes it seems to get stuck, forever animating the 
Is it possible to force my MacBook to forget and/or bring back the network password entry dialogue?
Best Answer
Open System Preferences (in your Applications folder).
Select the Network pane, and in the left-hand list of sources, select Wi-Fi or AirPort (this differs in different versions of OS X).
In the lower-right hand corner, click Advanced…. A pane will pop out, and in the list of tabs at the top, either Wi-Fi or AirPort will be selected (depending on OS X). This is the leftmost tab; if it isn’t, then click on it.
You should see a list of Preferred Networks. This is a list of all the wireless networks that your Mac knows about, and thinks that it knows passwords for. Select the network which has a changed password in the list, and press the little - button which is to the lower-left hand corner of the list.
Then press OK in the lower-right hand corner of the pane, and then Apply in the lower-right hand corner of the System Preferences window.
This “forgets” the network. The next time you ask your Mac to connect to that network, it will see it as a new network and throw up a password prompt for you. Once you enter the new password, OS X will treat it as before (logging in and connecting automatically for you).