Very first i have disabled this, after that i can't find the option to enable it
I'll supply the answer from my Samsung Galaxy S3, assuming a similar setting is located at a similar location on your Xperia. Look for Settings>Display>Touch Key Light Duration. On Samsung there are settings from Always Off to Always On, and timed settings in between. Choose one of your liking other than Always Off to restore lighting to the buttons.
UPDATE: Sorry if the settings are not exactly at that loocation. Here is a link to the User Manual
UPDATE2: I notice that the Xperia has a slideout keyboard. Sometimes the keyboard lighting is controlled by the control key lighting. Look in the same place, except for Keypad Timeout: Settings>Display>Keypad Timeout.