Sounds like you're trying to use a 60W AC adapter instead of an 85W adapter. IIRC, 60W is enough to run a 17", but not run it and charge the battery at the same time. So, first thing I'd do is make sure you didn't somehow switch the AC adapter with someone else's or even another laptop you may own.
If yours clearly says 85W on it, then take it to an AASP or Apple retail store, they can run a quick diagnostic on the adapter, and if it fails, replace it. If for some reason it's a 60W, then you're just kind of screwed unless you mixed them up between two laptops you own. You'll have to go buy a new AC adapter, since no AASP or Apple store is going to give you a new one because you were careless and lost the old one.