I would open that unit up and see what's in there. You could have burnt your motor or had something climb up in it. If you don't feel confident taking it apart, hire someone to come check it out for you.  http://www.newageair.com.au/ducted-air-conditioning/

It could also be the water pressure of your home. If you shut off the device like Colleen said, you should be able to open the door. Check the pressure before attempting to use it again though.