ok, first, what viewer are you using. This problem is usually because your puter and the SL server are not connecting up. First try TPing to a different sim. If that doesn't work then the best way I have found to fix the problem is this.
1. If you don't have the 1.23 SL viewer download it and then log in to a different sim, such as Pooley. To do this, on your log in screen go to edit, preferences and then check Show start location on login screen. You can then go down and type in Pooley.
2. change your bald cap. If you aren't wearing one, you can find one in your inventory. That seems to be what fixes it the fastest for me.
3, Under advanced (hit ctlr, alt, D to get it up) go character and test male or female, that will put you back to the default SL avi.
4. Now, you should be able to wear your original skin and avi. Be sure you let your inventory load completely before you sign out.
5. Now sign in on your regular viewer.
Good luck