This is happening very frequently with my Ubuntu 12.04 LTS. I am unable to see anything on the status bar(The ui elements which show wifi strength, battery status etc) and it remains blank. I am unable to do anything and then I have to do a hard reboot after which the problem may/may not persist. How can I rectify this problem ?
This happens when when your drivers are not compatible with your hardware.so u need to download the compatible ones.
first of all open the grub menu by clicking f6 or f7 when ur PC starts(depending on ur PC)and then open the OS in advanced-> recovery mode and then click on fail safe X mode and if it shows that you are low on graphics,cannot detect screen and devices then the problem will be mostly due to the driver.
Now depending on your hardware install the drivers from the internet whether its nvidia drivers or ATI etc.
Then go to the setting option->software and updates-> additional drivers->then selct anyone of the drivers and then click on apply changes and then restart your PC.
This will do your work if the problem is with the drivers.