Forks are easy if you know what you are doing, if you don't then you can really knacker 'em up so probably better to pay out and take 'em to the garage. By the sounds of it the chances are you've lost oil out of one of the tubes, maybe through old fork seals, maybe just not done properly in the first place. If you feel confident enough to take off the top of the fork tubes (can be done whilst they are still on the bike if you can get the socket in) then you can 'dip' the fork oil with a length of CLEAN wire, a bit like checking the oil dipstick on your car, to see if the levels are equal or not. Check the seals & fork bottoms (you should have a bolt/allen thing that holds the fork internals together) for any signs of oil, don't forget to lift the dust caps off the top of the fork seals. Also check the sliding part of the tubes are smooth and free of damage, scrathes and rust as these will all affect the performance of the oil seals.