Try rebooting your equipment. Turn your Sky+ box and broadband router off, then on again at the mains. If you're using On Demand wirelessly, reboot your Wireless Connector too.
Check the network settings on your Sky+ box are working: Press services on your Sky remote control. Press select to highlight the Options tab. Use the left/right arrows to highlight Customise and press select. Use the down arrow to highlight Broadband Network Connection. Use the left/right arrows to toggle the setting to ON. Press the green button to save your settings. After this please check the following: Press services on your Sky Remote Control. Press right to highlight the settings tab. Press select and scroll left/right to the network tab. Press select and check that the local connection and service connection are both OK and the IP address field is populated with a number and DHCP is ON.