visual editor not functioning

Try deactivating all of your plugins to explore whether any could be responsible. If none are, keep all plugins deactivated and switch to the Twenty Fifteen/ Fourteen theme to see whether it could an issue with your theme.

nope i tried right now friend by activating the 2015 theme,but no use

Try using your browser’s console feature to debug any JavaScript or HTTP error you could be getting when the issue occurs: https://codex.www.ads-software.com/Using_Your_Browser_to_Diagnose_JavaScript_Errors