This could be due to a number of reasons but follow the list below for a sequence of events to eliminate the usual suspects, we assume you are using broadband or cable for your conection.
1. Is the modem/ router switched on?
a. Check the lights on the front panel. Although lights and configurations vary depending on the manufacturer, most have a light to show
i. Power is on/ off
ii. Connected to a PC via RG45 Cat cable
iii. Connected via wireless connection
iv. Broadband or cable connection - usually flashing indicating broadband avaliable or steady indicating connection is made.
2. Does the broadband provider have a problem? Usually you look on the website of the ISP to determine if they have a problem. Naturally if you cannot connect to the internet this is not an option. You could phone a friend and ask them to check the website for you or phone the ISP directly.
3. Test to see if you are correctly connected to your modem/ router.
a. The light on the router is the first port of call (see 1. Above)
b. Try to communicate with the router by doing the following:
i. Click start,
ii. Select run
iii. Type CMD and press enter
iv. At the Prompt screen type 'ipconfig' and press enter then look for the default gateway address, which will read something similar to: '126.96.36.199' (numbers vary depending on your router)
v. Type 'Ping' followed by whatever your default gateway address is e.g: '188.8.131.52' and press enter
vi. If you get a reply from the address you can see the router/ modem
vvi. If you get a reply which has timed out or says destination unreachable you are not communicating with the router/ modem. Check cabling or wireless connection.
4. Check for Virus/ spyware