The way that the Frigidaire dishwasher is loaded can affect whether or not the detergent cup can open as large items in the front of the racks can block the dispenser. Frigidaire recommends that smaller items are set on the upper rack and larger pieces set along the left and right sides of the bottom rack and in the rear, but not in the front, which will keep the space clear in front of the dispenser.
When detergent, rinse aid or food residue gets spilled on the soap cup lid or switch but is not cleaned up, the stickiness can cause the lid to get caught. You should clean the dispenser, lid and switch on a regular basis with a clean, damp cloth to ensure proper performance and to help prevent dishwasher soap from clumping. Follow by drying the cup with a towel so that any remaining dampness does not create further sticking.
While damp conditions or residue can make the dispenser door stick, another culprit is old detergent because old detergent does not dissolve completely and may also cause the door to get stuck to the partially dissolved (and therefore sticky) soap inside the dispenser. If the detergent is several months old, discard and use fresh dishwasher soap that is stored in a cool, dry area to help prevent any further clumps.
Another possible problem is if the door switch is stuck on the dispenser cup. Try toggling the switch back and forth to see if the lid will release. If the lid remains closed, push down on the small white lever located on the lower edge of the cup to manually release the cup. Stopping the dishwasher cycle before it is complete may mean that the detergent has not yet been released and the door will remain closed. Close the dishwasher door and press “Start/Cancel” to resume the washer, or start a new wash cycle if the previous cycle was accidentally canceled.