Upon startup, the screen has a message that reads: Memory Card Locked! How do I unlock it?

There is a movable plastic slide switch on the side of the memory card, and it can be moved to the lock postion. Slide it the other way to unlock it.
If it's already in the unlock position, clean the contact legs with a qtip and alcohol, or a pencil eraser.
Other than that, the memory card could be bad. Try another one, if you have one.
If nothing works, the unit may have to be serviced. Take it into a local repair shop for an estimate and assessment, or contact the manufacturer's support site.

You didn't specify the make and model of your camera, so I'm assuming it uses SD cards.

Look at your memory card. SD cards have a slide switch along one edge. The position farthest from the contacts locks the card, protecting it from writes. The position nearest the contacts unlocks the card.