From the inside, before removing the whole panel, you will see a plastic cap above the locking mechanism, if you are careful, you can activate the latch to open it, do it, and then lube the hell out of it with penetrating oil as ford green said, also spray inside the key cylinder and around the button, activate the latch with the screwdriver a few times, as well as the button, the try it. If it works, that was your issue, just add some thicker oil or lithium grease or something that will last and won't affect the plastic inside. If it stays the same, well, then take the panel off and have a go at it, there are some plastic rod connectors that fail all the time, also the armature around the button, etc.