Auto's are very definite pieces of engineering, they either work or they are broke, not really any middle ground with them.
First of all, check your fluid level, this is the most problem causing thing with auto's, on the gearbox dipstick, there may be a set way to do this, such as having the 'box at operating temp and then moving through all the selections whilst stationery and then checking the level while the engine idles.
This is how it is checked on my Jeep auto and on Range Rovers I have had, but it may vary between cars.
Make sure the 'box oil is a cherry red colour and not anything else, also have a sniff of it to see if it smells burnt.
It may just need a fluid and filter change.