So I fell asleep during some quest, and had Ria as my follower. When I switched on today, she's nowhere to be found. Not my house, not in Jorrvaskr, and my load files date back too far and I'll lose my progress. I have fast travelled several times, and "waited in game" for over a week game time. The worst part is, I can't get a new follower, everyone tells me it seems I already have someone. So what do I do now? I'm using a PS4.
Since you use a PS4 this problem is a bit harder to fix than on PC were you have the console to fix almost any bug.
You have really only 1 option if you don't want to lose any progress:
backtrack to the position where you lost her and look in the surrounding area. Followers can get stuck and not be able to teleport back to you even if you fast travel. There you can try to either Fus Ro Dah her lose, or just kill her.