trans problem 94 seville

I have 94 seville sts and the other day i was pulling out from red light and i heard this pop noise and the cars rpm started racing up.I let off the gas a little bit and it took off,now when the car gets to about 30 or 40 mph the car is not going into overdrive and rpm's race up.the car will shift to 1st,2nd,3rd gears just fine but not 4th overdrive.
Anyone know what might cause this or is the trans shot or maybe broken gear?

The trans lost the o/d spline/hub assy, needs to come out for a repair.