Best of luck finding your adorable kitty!
Have you checked in every possible nook and cranny in the apartment? I thought I lost mine a couple of years ago, but he was hiding in a place that I definitely didn't think was kitty-sized.
Also check inside your building, knock on everyone's door if you think she got out behind you but could still be in the building (heck, even if you think she could have left the building).
You'll find some more tips here on a thread about it when my kitty hid so well, I thought he was gone:
Lost Cat - What Now? [Basel]
Good luck, and let us know how it goes!