1. Have you tried turning it off and back on again?
- Both Computer AND Router (if applicable)
- Yes this sounds dumb, but you'd be surprised how often it works.
2. Are you behind a proxy?
- Open IE
- Internet Options -> Connections Tab -> LAN Settings
- If there are any settings there, make sure your other browsers also have the same
3. Any software interfering?
- <insert random vendor> INTERWEBS SECURITY PACKAGE may be interfering with it, if your running any check if its blocking your browsers.