Visual/HTML is missing from WP post editor!!

Hello,
I have self-hosted WP blog. A very quite odd thing happened with me. Just few hours ago, I’ve come to know that the ‘Visual/HTML’ option is missing from my WP post editor. I can do bit with HTML but it’s not good idea for long time.

I’ve attached my screenshot below for better understanding. Why I can’t see that? I’ve checked with my friends, they are WP users too.

Anyone can tell me how to get it back? Please!

Thank you!