You arrive at the hotel only to be told that your guaranteed reservation wasn’t guaranteed after all and there is no room for you.
Ask the hotel to find you a room at a comparable hotel at the same price. Do not leave until you have a room. Make it clear to the management that you are not leaving until they find you a place to stay. Which might not be an easy task at a busy resort during the high season.
Before you go anywhere, get a letter in writing from the hotel that it was unable to provide you with a room even though you had a confirmed reservation. This can be useful if the hotel isn’t able to find you another place to stay or the replacement hotel turns out to be a dump and your vacation or business trip becomes a nightmare.
In addition, the hotel should provide you with transportation to the new hotel…assuming they find you one.