Why won't your automatic transmission go into gear?

Answer

Possibly because the gears in transmission are in need of replacing. Take it to shop and have them check it out. Just had mine done and that was the problem.

You may just be low on trans fluid
Leak in hydraulic circuit, causing transmission to slip?
It's either out of Transmission Fluid, the valve body is bad or the clutch pack is worn out. In any case, you probably need some serous transmission work if adding fluid doesn't solve the problem.