When I first connected my iPad2 I had a number of problems, the third being that it would not accept our network password (for the Homegroup, which our computer is able to display so I knew it was definitely the right password).
I tried the suggestion to reset the network (Homegroup) password, but that did not work.
In the end I had to not only reset the Homegroup password, but I also had to reset the Wireless Router password.
WARNING: BEFORE you do this, make sure that all computers, iPads and iPhones that utilise the wi-fi are on (not asleep) and as soon as you make the change, reconnect using the new passwords on all devices that you own. Pain in the bum, but it worked in the end.