On top of the excellent advice from my learned friends above can I add this extra info. If you cannot get the original or near save of the property then not all is lost.
I often find standing on a solid cube (one single prop with collide ability added in the State Tab) helps in cases like this. Many people fly instead but as a personal thing I don't like this so much. Either way, stand or fly, get above the non solid object and add back collides, but just make sure that it is the object that had the collide in the first place. Don't go fast, take your time, be patient and I'm sure you can undo the problems. You're not the first person who fell through a floor they accidentally took the collide off.
Nor will I be the last If you know the floor is at level 5,000 then put a collide set cube at 5050 or something similar then make a port to it. Repair the beach floor, jump on to it, test then delete the cube (
take off the collide before a delete). My friend calls it "power of the cube" because I use them a lot. The most often use is finding the XYZ settings of a far off area so I can port there. I place a cube, take a reading of the XYZ in the POSition Tab, delete the cube (IMPORTANT YOU DO THIS) then link to it. With maybe a small collide set floor under it. I usually use the fishtank glass lid for a temporary floor as it's easy to spot with being blue. If you use the floor idea instead then it's best to add a little to the Y axis of the reading. Otherwise you may fall through this temporary floor or get stuck in it.
Finally, the list icon shown above can save your bacon most often too. If you know what prop the main floor is then look for all occasions of that prop in the list. Open the first one and open the POSition Tab if not already open. The Y axis will be one of the highest settings in the list so it should be relatively easy to discern which is the solid floor of the beach. You want to appear above that on a temporary solid floor, cube or other prop to put in repairs. Just watch out for -numbers as they change what determines you are above.
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