i had similar happen the night before graduation, had my game saved at about 6 pm that night, at 7:30 pm the game froze up like others mentioned, sims in the middle of an action kept playing that action even in build mode (one sim was juggling a soccer ball, two others were playing violin) and time wouldn't advance. had to kill from task manager, since couldn't travel or save (tried each one on a different attempt)
i did find a bit of a workaround for it, though. played another household that has nothing to do with university until about 7:45 pm that night (so, a bit under two hours game time) and then swapped back to my uni sim. things worked fine, night went okay, graduation went off without a hitch...then the game did the same freeze up at 3 pm that day, which was not long after i stored all the portable belongings in a chest and placed that chest in my sim's inventory... am no wondering if that was the key, since i did that several times the night before when it locked up. maybe putting a mini fridge full of food inside the chest and then the chest inside inventory... i dunno...
editing to say that i reloaded the post-graduation save, didn't store anything at all and the game didn't freeze up at 3 pm. i'm now trying a few different things to see if i can replicate the freeze but so far, my initial guess is it has to do with hungry sims trying to reach the food in a container in the sim's inventory (or in a container in a container in the sim's inventory) and the game not knowing how to handle that
