OK, just keep calm, this has happened to me many times before with hamsters. The first thing to do is go into every room, and be completely silent, you may hear her scrabbling around. If not, does she respond to her name? Try calling her. If you can't find her at all, then leave food on the floor that leads to her cage, and keep it open. More often than not little animals try to return to a place they know is safe to sleep.
I hope that you find her and she is alright, keep us updated if you can.
Bookmarks