If you have configured your computer to Obtain an IP address automatically, sometimes such as after you change the IP address of your router,  you need renew the IP address of your computer to access the network/router.
 
For Windows 2000 & Windows 2003 & Windows XP
 
Step 1
Click Start->Run, type cmd and press Enter.
 
Step 2
Type ipconfig /release at the prompt window, press Enter, it will release the current IP configuration.
 
Step 3
Type ipconfig /renew at the prompt window, press Enter, wait for a while, the DHCP server will assign a new IP address for your computer.
 
For Windows Vista& Windows 7
 
Step 1
Click Start on the task bar, type cmd into the search box and press Enter.
 
Step 2
Type ipconfig /renew at the prompt window, wait for a while, the DHCP server will assign a new IP address for your computer.