Lock Button Not Working Properly....HELP!!

My lock button a couple of days ago stopped working. I will admit that about 5 months ago the phone was dropped by accident and a little crack showed but since then the phone as worked fine, like brand new. I am able to hold the lock button and the home button together which resets the phone after a while but I am unable to just lock it so I have to wait for it to go to sleep itself. Any suggestions on what I should do? The phone is about 13 months old.

Any comments would be much appriciated,

Many Thanks! 🙂