iPhone 4S won't connect to WiFi, but iPod and Mac are connected.

I was on my iPhone 4s on the internet absolutely fine about 20 minutes ago and suddenly the WiFI stopped working? I tried to reconnect and it keeps saying 'Unable to Connect'. My iPod and Macbook along with my mothers iPhone 5s can connect and are working absolutely fine, but not my iPhone 4s? I've restarted my phone twice and forgotten the network but still nothing! Help I'm getting very stressed!