I've had that happen a few times. Usually Sims 3 doesn't show up, but as I don't play that anymore, i was gratefull that sims 4 was still there. But as I play off line, usually when I hit that, Sims 3 shows up then.
It's all very frustrating, and scary!
I have learned though, that restarting the app brings them back. Not good if you're not aware of that.
And just for thee record, I just signed in and Sims 3 again was missing. I went off line and it was still missing, and then I went back on line and it was back again! ?