please try not to panic, as said above, it is most likely that you have gone through the plaster and hit brick, it feels quite different when the hammer strikes a nail that is trying to push into a brick than the soft plaster that is over the top of the brick work.
again, as said above, if you could post a picture showing the wall from a good distance away it would help us to estimate the likelihood of you bouncing a nail off some capping (protection for electrical cables)
if there are no obvious sockets or light switches directley above or to either side of the place you are trying to place the nail, try putting another nail in the wall. put this 6 inches (150mm or 15cm) above or below and 6 inches left or right of the original place you have tried.
if you find the same thing happens then it is most probable that you have just got to the depth of the brickwork.
take the new nail out of the wall, pop a bit of polyfill over the hole and either hang your picture on the existing nail, bang it in further (this might be a LOT harder than you have hammered so far) or put a shorter nail or picture hook up.