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 8.8.8.8
gives the result Network is unreachable
.
The solution offered by drewski22785 below have solved my problem!
Thanks to everyone who gave advices!