I beleive you already stated the answer to your problem. "I havenít changed anything Iíve been doing". When you hit a "plateau", it's usually because your body gets used to the routine, and has reached a point where it no longer needs to adapt to the strain you are putting on it (the way your body adapts is by building muscle). Change your workout around. If you usually to low reps with high weights, do a cycle of higher reps with lower weights. If you usually use machines, switch to free-weights. If you normally do bench with the bar, switch to dumbbells. Change your program around to force your body to adapt again.