well the main point would be to know what changed those last days:

- update of the server: PHP/MySQL/Apache?
- installed/activated plugins in limeSurvey, apache, etc?
- changes in global settings?

Also, what version of PHP are you using? PHP 5.3 makes a lot of weird unpredictable problems (most buggy PHP version ever).

i have two limeservay databases and i tried to connect on both dbs by changing the dbname from application/config/config.php and same error is shown

So now, just try the contrary: backup everything, install a fresh up to date LimeSurvey (2.06LTS or 2.5), connect it to the old DB, and see if it solves the problem.