A true frozen shoulder (or adhesive capsulitis) can be a very difficult thing to live with, primarily due the pain and discomfort it causes and secondly because it can take a long time to fully recover (usually up to 18 months).
It occurs when the capsule of the shoulder joint becomes thicker and inflamed and then essentially ends up sticking together, which can cause pain & severe restriction of movement at the shoulder. There can be a few causative factors but most commonly its due to previous injury to the shoulder, prolonged immobility as well as an increased risk if you have diabetes.