Can't get into top or bottom gears

[dougwo]

I dug out my bike after a 3 year hiatus from riding. (slap me now). I noticed that my rear derailleur (Shimano XT) won't shift into the top three or bottom three gears but shifts smoothly between 4 and 6. Is this because my cable has stretched or slipped? My h and l screws are about 3/4 of the way out. My cable tension adjustment is at about 50%. What is the procedure for setting the correct cable length before going through the fine tuning modes? Or am I barking up the wrong tree?

[cyclerUK]

Pull on a bare part of the cable (either downtube or toptube) with your fingers while turning the cranks. [You could turn the bike upside down and rest it on the saddle and bars to do this if you haven't another method to support the rear wheel of the ground?]
This should move the derailleur through it's travel. Do the same again only this time operate the gear changer. One or the other will be probably be jammed. The other probable is the cable has rusted especially if it goes under the bottom bracket.
So:-
Check cable for jamming.
Check gear changer.
Check derailleur.
I doubt the cable has stretched much so cable adjustment should only be minimal.

[cyclerUK]

Sorry but as an explanation of checking the gear changer.
Pull on the cable but not turning the pedals.
Tension the cable with your fingers to simulate the spring pressure of the derailleur.
So pull the cable and operate the gear "trigger" and it should click from top to bottom or vice-versa