Can you hear the CD spinning? It's VERY possible all it has is a dirty lens. You can either 1: buy a lens cleaning CD or 2: take the unit apart enough to access the lens in the CD mech, then take a q-tip and some rubbing alcohol, dip one end of the q-tip in the alcohol and wipe the lens, then use the dry end to wipe off the excess alcohol (i.e. dry it out).
If the above doesn't work, it's a mechanical issue (bad pickup, most likely). Unfortunately, Jensen head units are fairly well-known for having CD/DVD players that go bad quickly.