What cirino.ed said in the previous post about "cupping" is correct most good deck builders will be careful of the grain of the wood to lay it cup side down. Decks should also always be screwed down NOT nailed as the wood dries the nails always have a tendency to pull out allowing the wood to buckle ( I saw a deck yesterday that over a space of 24" the deck board has buckled upward 8").
Is the deck treated lumber?, Java_bob has a great point about the type of material and the quality, I always tell this to everyone.....There is cheap wood and there is good wood, usually the job price is a good place to start with quality comparisons.
As for fixing it, have the deck and handrails screwed down if it is not already, (I doubt it is if it is buckling so badly). Have any very bad pieces of deck and handrail replaced, have the deck power washed down and after it has dried ( couple of good drying days) re coat the deck with a good quality sealer.
I like Cabot's (not cause it comes from where I once came from
) but because I have had very good success with their products over a long time span. Most of the upper end sealers are pretty good.
Sealers are like paint, you get whet you pay for.