Skrachfays

I think this is an iMac/Yosemite or Steam problem, because a similar issue has just cropped up with Sunless Sea, after it worked perfectly for 2-3 days. I have played the game perfectly fine to about halfway through, but now, when I load the game, the music starts but there is no video until, after waiting for ages and eventually clicking the mouse, the top 2-3 cm of the menu screen appears but no more. The only way to escape is to pull the plug on the machine or mash all the keys until I get the desktop back. I removed the local content and downloaded the game again. It worked fine the first time, then the old problem resumed. iMac (21.5-inch, Late 2013) 2.9 GHz Intel Core i5, NVIDIA GeForce GT 750M 1024 MB, 8GB RAM, approx 5GB free (attested by MemoryClean). Any ideas?