A 'slipped disc' is usually the most common cause of sciatica, but in some cases there is no obvious cause.
A slipped disc is rather a misnomer. The discs between the 24 vertebrae in the spine have an outer case, containing a gel like substance and if it is ruptured or split, due to injury, this substance seeps through the outer casing causing the vertebrae to compress and, possibly, trap a nerve. The pain is, therefore, felt locally and anywhere that is attached to that nerve.
Other, not so common causes include: spinal stenosis (otherwise known as narrowing of the nerve passages in the spine), a growth in the spine, infection or injury.
Usually the disc will shrink and the pressure relieved but this can take up to 6 weeks. Rest for the first few days but gentle exercise to keep the back supple and mobile will aid recovery.
Avoid any activities that could aggravate the condition such as: lifting, reaching and sitting down for a great length of time.