It sounds like the finish has oxidized or worn. If the condition of the floor is otherwise ok, I would rent a large floor buffing machine and "screen" the floor with a light abrasive "screen" to remove most of the finish. Vacuum and then re-urathane the floor. If you use the water based stuff then you might even be able to get two coats on in a day. Use a very gentle screen in between coats, vacuum and put a total of 3-4 coats down and it will look brand new. I stay away from high gloss and use the next duller (satin?).