Could anyone help me in finding the equation for rotation around ANY vector?? I need it for my camera in OGL...

Try quaternions. They have a good tut over at gametutorials.com

http://www.gametutorials.com/Tutorials/opengl/OpenGL_Pg4.htm

or try ultimatgameprogramming.com

http://www.ultimategameprogramming.com/Tutorial.php?category=OpenGL&page=3

dimebag

OK and is this possible that my view cannot rotate more than 90 deg up and down because of the "up direction"vector?? Can this be the reason??

You should rotate your "up" vector along with the camera position.