Check that the boot release lever isn’t stuck in the open position, ie pulled.
Feel behind the chrome (?) strip above the number plate, and if you can’t feel the lever then it’s stuck open. Push it back into position and the boot should open from your remote or the button inside the car (on the door).
To clean it up, open the the boot and stop it before the tailgate reaches the fully open position - stop it when you have a good view of the lever. Give it a good squirt with lubricant and work the lever back and forth - be ready with a rag or kitchen towel to catch any excess.
Replacing the boot release lever is the best fix but you can clean it several times to extend the life of the existing lever.