...for no reason at all...
eg : "I can connect to a wireless connection / modem but i can't browse the internet or ping an internet address.."
try this : reset your IP address :
1. go to start --> run --> cmd = to open your command prompt window
2. type the following :
netsh int ip reset.log
press "enter" and type
netsh winsock reset catalog
3. Restart/reboot your computer and see if you are able to browse the internet.
It worked on my machine.
or below is another option to renew the IP address for our computer
in command prompt window :
check2 : IP Anonymiser