98 malibu wont get out of park

i replaced the shift knob and that was not the problem...when the problem first occured it seemed to happen in the mornings (cold) or when i hadnt driven for more than 8 hours...now, no matter what time of day it wont get out of park but it still starts though...if it isnt the shift knob what else can it be?

Your car probably has a shift lock deactivated by the brake pedal. First check brake lights to see if they are working. The brake pedal has a switch that activates the solenoid on the gear shift to release lock.