Why might a transmission not shift?

what type of transmission? for 4L60E trans, try changing the A & B solenoid before giving up on ur current trans, these are located in the pan, they are black and sit side by side, a spring clip holds them in place. cost bout $50 Answer

If it has a modulator valve gone bad - this will prevent it from shifting. Call Auto zone and ask if there is this type of valve for your transmission.

Check the fluid level. Check for trouble codes with a scanner, Check shift linkage/vaccum lines.