Greetings Nish,
I've had a few experiences with what I think is similar. One method that worked.. was the avatar was able to log on, but not to the region where her avi was. She logged onto a different one, then traveled to the one that her ghost was stuck on.. it immediatly poofed when she arrived, and all was fine.
Another incident was a buddy crashed while riding a horse, and was not able to log on at all. Fortunately, I am an estate manager for that region, and was able to 'eject' him. (I found his avatar and horse frozen). A third similar incident, I could not find the ghost, but restarted the sim, and it cleared it. So I don't know if it was a sim problem or not. Perhaps you can get in touch with an estate manager of that region.
Good luck!
Riv..