Why after two years would it stop opening all the way?

Sounds like there is an obstruction in the track, the track is loose or misaligned, or the garage door spring is broken.

Here's what I'd do:

Let the opener raise the door (the 6 inches or so)
When it stops, does the light blink? This would indicate the opener thinks the door is too heavy or too tight.
Pull the emergency disconnect cord on the trolley
Like this (http://www.naturalhandyman.com/iip/infgar/i/emergencyrelease3.jpg)
Make sure your feet, spouse, pets, children are not underneath the door, as it may come down fast!
See if you can raise the door by hand
If it feels really heavy from the beginning (or fell rapidly in the step above) this indicates a broken or weak spring.
Feel for any tight spots while raising the door
If it raises easily for some distance and then gets tight, look for a track obstruction or misalignment.
If the door raises easily all the way up by hand
Look on the opener motor unit for a couple of screws labeled something like "Close (Down) Force" and "Open (Up) Force". Turn the Open (Up) screw in the direction indicated to increase. Do this in small increments (say 1/2 turn) and test the opener in between each adjustment.
NOTE: Before testing, re-attach the door to the trolley assembly


Hope this helps,
WallyH

I agree but add that most doors have a means of telling the door how far to travel. Weight can stop a door but so can a travel limiter that may have moved on the track. Did you install this opener and do you still have the installation instructions. If not what is the model number?

Problem was solved dwhen I tightened the chain which was hanging outside the track guide.

Rather than giving me a reddie it would have been nice to tell us your chain was hanging outside the track. Leaving out obvious info and not answering my questions left me in the dark. I was trying to diagnois your problem and had not ye even given an answer. Did you even read the end of my post? "Please do not "rate" my questions., wait for my answer"