keyboard letters not working suddenly.what is the problem? but it again works when i restart!

Try using safe mode

http://support.apple.com/kb/HT1564

The reason being “3rd party” applications could corrupt system files and in safe mode they are disabled.

If Safe Mode doesn’t work after testing, creating a test user to see if it’s user specific or system wide issue.

Creating a New User

https://support.apple.com/kb/PH25796?locale=en_US&viewlocale=en_US

Try to find a external USB Keyboard which this method could help in finding if it’s hardware related issue or not.

Your problem, hardware or software, may not be exactly the same as that of the original poster of this older thread, and it can be very confusing for everybody if we try to answer more than one question in each thread, which can also result in you applying the wrong advice to your particular problem.

In order for us to give your problem our proper attention to try to solve it, would you kindly start your own thread, describing the trouble you are having in the fullest detail, including completing your details to show what Mac or iDevice you are using, what operating system, and what version of the application in question. Please remember to post in the forum relevant to your hardware or version of OS X. A full list of all the support forums is here:

http://discussions.apple.com/index.jspa

I had a similar ******* problem.

I finally solved it by doing the following:

Go to: Settings -> Accessibility -> in the left pane scroll down to Trackpad & Mouse -> remove the check next to the option "Enable Mouse keys".

Keyboard Doesn’t Work

Keyboard Keys Do Not Respond