Wow, I just came on here to post about this same problem. This seemed to happen when I got a new computer and began installing a new printer. Not sure if it was the computer or the printer, or something else entirely.
I tried rebooting the router and the phone, but no dice. My only solution was to go into Wifi Settings > Advanced > Pick 'Static IP' > enter all the IP/MAC Address info. You get into advanced by pressing the menu (lower left button) button on the Wifi Settings page.
If you're using Windows you can get all your IP info by launching the command prompt and typing "ipconfig" (no quotation marks). Here's how to do that in case you have never used the command prompt:
Go to start menu > In the Start Menu 'Search Bar', type 'cmd' (no quotations) > Press enter > the black CMP prompt should pop up > type "ipconfig" (again, no quotations) and all the info you need will be there.
BTW, the "Netmask" on the Galaxy menu is the same as the Subnet mask listed in the command prompt.
I'd like to know of any other solutions, or why this happens.