Have you been able to connect to the same network and router with this phone before (it sounds like you have, just double-checking)?
If so, power off the router, let it sit for at least 30 seconds and then power it back on. After that, make sure it still works by connecting a computer or some other device.
If all this fails, flush your phone’s RAM: power down, take out the battery, hold the power key for 30 seconds, re-insert battery, power up.
If this still fails, take your phone to another Wifi network (friend, family, work, coffee shop, Starbucks, McDonalds).
Report back here.