Well, shizzy.
Suggestions:
*Be sure ALL your drivers are up to date. I know NVIDIA released a new update a few days ago.
*Check Cloud to see if there is any old files, If so, swap the files.
*Go into Steam.
Right click on Planet Zoo.
Go to Properties.
Be sure Steam Overlay is NOT checked. If it is, uncheck it. (You be amazed how this can screw up your game at times).
*Run you game.
When Planet Zoo is fully loaded (at the loading screen). Alt Tab
Hit "CTRL ALT DEL" this should bring up the Task Manager.
On the very first Tab -- Processes, you should see Planet Zoo running.
Right click on it and select, "Set Affinity." Every box should be checked. If not check the empty boxes. Hit, "Ok."
Right click again on Planet Zoo (in the Processes Tab). This time select Set Priority. A cascade window should open. By default "Normal" should be checked. If is not check it. IF it is checked, then change the priority to "High."
Exit out of Task Manager, return to you running game and load you game file.
And if you want something more advance try here.
Options you could do to prevent this from happening again:
*Install your game on a different drive. (Mine are on D). The reason is some games do not do well on the main C drive because of other programs running -- like Firewall. While the game may run fine, sometimes weird hiccups like you are having will randomly occur.
*ALWAYS have at least several saves of your current progress. Never have just one save. I save a lot. This way if I am unable to recover the current file and/or fix it for whatever reason, have a backup. Sure, I may lose 10mins or even an hour, but it beats having to start from scratch.
*You canr ensure your progress is never lost by setting up a save folder system. That involves creating a folder (in whatever directory you choose) and naming it the game you are playing. Then within that folder, create another folder and entitle it "Play-through 1," or whatever you choose. Then locate your save files from the game. Copy the files you want and paste them into the "Play-through 1" folder. You can even go a step further and use an USB drive for the location or heck both!
Phew! Good Luck! And I hope you get this resolved!