I had asked for some additional information and have not heard back.
When someone is having pain or difficulty in lifting the arm, it is most often related to a problem affecting the rotator cuff muscles or tendons. Although the deltoid muscle also lifts the arm, it is primarily involved once the arm has already been lifted to 90 degrees, while the rotator cuff is primarily involved in lifting the arm to 90 degrees.
If there has been a recent injury, there would be more of a concern about a possible tear. Otherwise, the most common cause of rotator cuff problems is degenerative changes within the rotator cuff or inflammation of nearby structures that is pressing on the rotator cuff tendons.
If there has been a recent injury or the symptoms has been of short duration, it would be reasonable to rest the shoulder. Ice would be appropriate, particularly for the first few days. After the first few days, then using moist heat becomes more important.
It also may help to use an over the counter anti-inflammatory medicine, such as ibuprofen or naproxen. I ask about other medicines to assure that there is no potential adverse interactions, either with the medicines or the conditions that they are treating.
There are also several exercises that can help with rotator cuff disease. These exercises are typically not done until past the acute phase, but they can be very helpful for management of chronic disease and recovery. You can review exercises at http://orthoinfo.aaos.org/PDFs/Rehab_Shoulder_5.pdf. Until the pain is improving, it would be better to limit the exercises to stretching exercises, such as the pendulum exercise on page 2. As the condition improves, then strengthening exercises can help.
If these home interventions are not helpful, then it would be better for you to be seen by your doctor.
If I can provide any additional information, please let me know.