Wear and tear may cause spinal discs to get out of place. 

When the inner cartilage pops out through the ring that usually contains it, a nearby nerve can get compressed.

The irritation from the pressure causes intense pain (sciatica) but usually resolves when the disc pops back in (or gradually moves away from the nerve which can take a few weeks).

If the pressure is bad enough to cause nerve dysfunction (weakness or loss of sensation), or the pain not improving over several weeks, imaging (MRI scan) may be needed to take a look at the situation.

Depending on what the MRI scan shows, more time will be given for the disc to heal itself, or endoscopic surgery may be needed to take the pressure off the nerve.