If you are referring to the button in the middle of the boot plinth then that is another boot release switch. It is a push button switch. If it looks as though it is stuck in then try to release it with a watchmaker’s screwdriver.
With reference to the key lock, how far can you turn the key when inserted. If you’re not able to turn far enough then it won’t unlock. Try WD40 and then working the lock, it may be gunged up through lack of use.