Took a break, now my old worlds are greyed out and unplayable.

I took a break from the game and when I came back all of my old worlds are greyed out and when I click to play them the game crashes when it says "building terrain". I can create a new world and the game will work fine but I cannot load any of my old saves from about 6 months ago.

I will include a screenshot to show this.. the world in colour is one I created to see if the game still works and it does, i just cannot get the greyed out worlds to load at all without the game crashing.

Any help would be appreciated, ty.