no hot water?

I am not getting any hot water at all from my bath tap,the cold is fine,also the hot water tap in the sink is fine,its just this one.it did work but gradualy got worse,to the point when it would only trickle out.now when you turn it on,a miniscule amount of water comes out then thats it.I believe its the original bath in the house and id say about 40yrs old,but thats just a guess.someone said it could be the washer ?help please.