Priapism is painful and can result in long-term damage to the penis
In many cases, the cause of priapism is unknown, but it is thought that it could be due to nerve damage or a side effect of some types of medication, such as drugs for erectile dysfunction. It may also be caused by conditions that affect the blood, such as sickle-cell anaemia or leukemia, or by an injury to the penis or genital area.
If it's been less than four hours, then you can try to get your erection to go down by taking gentle exercise such as jogging, or taking a warm bath at home. It can also help to pass urine (this may be easier in a warm bath), while painkillers might go some way to relieving the discomfort priapism causes.