There are multiple solutions to this problem. First look on the ceiling around you (if it is a network at school) and what color is the light? Is it green? If so, the WiFi is down. If it's blue then it is a problem with your computer.
This router light is blue, so the internet is working.
If the light is blue, the first thing I would try is to disconnect from whatever WiFi network you are connected to, and then reconnect.
If it still isn't working, then try turning off the WiFi, turning it back on, and reconnecting to the network.
If that doesn't work, then restart your computer. Please click on the restart link for instructions on how to restart.