I have wireless Internet access and it worked well on Ubuntu 12.04 LTS until today, when I installed updates from the Update Manager. After the reloading of the computer the sign of Internet disappeared and the Internet connection stopped working.
I went to the Network section of the System settings and got such a message:
The system network services are not compatible with this version.
ping 18.104.22.168 gives the result
Network is unreachable.
The solution offered by drewski22785 below have solved my problem!
Thanks to everyone who gave advices!