Frozen Shoulder has no known cause.  It is a condition where medicines have little effect. Even surgery is not very effective.  The only real treatment is a tailor made and graded exercise program for your shoulder. You may need assistance from a physiotherapist and some sessions of manual therapy will be advisable.  The earlier the treatment starts the better the results.  Unfortunately the normal period of recovery is up-to a year. It's a long process but exercise is the only real option.