The most common muscle I find with trigger points (knots) and adhesions in people is my friend LEVATOR SCAPULAE. Reasons that this muscle becomes overused and agitated: Head forward posture (from typing, not sitting or standing up straight), sidelying or face down sleeping with the neck at an angled position, carrying something heavy over one shoulder (purse, computer bag), or having head tilted to hold something in between your shoulder and head (office phone, cell phone, etc), stress also agitates this muscle as it causes us to take shorter breaths that elevates our shoulders and causes the muscle to contract.
Levator Scapulae can cause a lot of pain, and also tension headaches. You will want to treat it by removing the cause of hyper tension (posture related, sleep related, stress related or appliance related). Often a slight chin tuck will improve that head forward posture. Then you will definitely want to stretch it. Two times a day holding the stretch 30 seconds or 6 deep breaths.
Massage therapy and self massage: Using a tennis ball or golf ball against the wall or floor. You can bring it into the lengthened position and place the ball on the upper/inner corner of your scapula, by using your body weight you can apply more pressure. You will have a good idea when you are on the muscle by the level of familiar pain it will cause, once you find that juicy spot stay there until the pain diminishes (can take up to a minute or longer).