The valve is called the "angle stop" and they do go bad after a few years, requiring you to replace them. Check to see that you opened it all the way and THEN back off the handle about 1/8 turn. Never lock it in the full ON position . . . this allows movement in the future.
Your "hissing" sound is due to a small leak inside the toilet itself which requires an infrequent re-filling. Usually it is an old flapper which costs <$10 to buy. Just replace it and the tank will be sealed again for a long time!
Good luck with your repair.