I was locked out of my TWO only sandbox games that I had spent hours on. I couldn't access the first one as soon as I'd restarted the game, but was able to access the second one a few times, then all of a sudden the 'requires hospital pass' issue shows and it stopped allowing access - I'm guessing a certain item I placed triggered it but no idea what item!
Luckily I've just managed to access my sandbox saves again... I went to the 'Sign Up' option, scrolled all the way down to the hospital pass section and signed in and I am now able to access them both. It's the same thing I had done multiple times before but for whatever reason this time it worked! So I guess just keep trying....