Not sure why but hardware failure is a possibility.
Why does my laptop hang after update & upgrade ubuntu 64bit 12:10
To test your system I suggest running Memtest 86; it's a good diagnostic tool-
Also, there is the chance that your
A driver installed for your Radeon card may help-
AMD Radeon ® HD 6290 Card may need a driver
To find out which Radeon card you have run the command " lspci" in a terminal and read carefully thru the output to find your card # Than visit the Radeon website.
Hope this helps