The same problem occurred to me also..but it happened after upgrading all the packages to latest version.
I have updated my kernel to the latest kernel as described in the post(log on to the link given) and it worked for me.
http://ubuntuhandbook.org/index.php/2015/02/upgrade-linux-kernel-3-19-ubuntu-14-04/
Problem fixed.
The same problem occurred to me also..but it happened after upgrading all the packages to latest version.
I have updated my kernel to the latest kernel as described in the post(log on to the link given) and it worked for me.
http:// ubuntuhandbook. org/index. php/2015/ 02/upgrade- linux-kernel- 3-19-ubuntu- 14-04/
Problem fixed.