Front Derailleur won't shift to Low gear
 

It's a Shimano RDX shifter & a Deore LX front derailluer. It's stuck in the high gear & will not shift back. Is there anything I can try myself before taking to the LBS? Thanks.

Does the chain ride smoothly in the high gear?

Yes.

Probable causes:
1. Bad shifter
2) Mis-adusted fd
3) Dirty cable/cable guide under bottom bracket

I would think that possibly the High Limit screw may need to be backed off slightly to allow the der to release. The cable tension is probably too high and the der is getting "jammed tight" against the stop.

If it was working OK previously, I'd address the cable issue first. Limit screws don't get out of whack. Easiest thing would be to disconnect the cable and manually check the cage throw, check that the cable is being properly released and then check thecable itself.

Try releasing the cable from the derailler then try the shifter without cable tension. Go gently .Usually it is jammed in the lever assy from what I call "dry shifting". "Dry shifting" is operating the shifter / derailler mechanisms when not on the bike riding. Usually it happens when the bike is not being rode and someone tries the shifter .
Be gentle, The levers are not happy with aggressive forceful action.