It seems that no matter how often Linden Lab squashes a bug, sooner or later it comes back again. The latest one is the old problem of "ghosted" avatars.
By this, I don't mean that your avatar looks like a cloud. That's a different problem -- while you may look like a ghostly cloud of vapor, the technical term for it is "bake fail". Your textures have not downloaded successfully.
Ghosting is another matter. You crash, or log out. Then the next time you go to log in, you get an error message: "The system is logging you out. Please try again in a few minutes." If you keep getting this message after two or three tries, you are ghosted. Your avatar is still in world, but your computer isn't connected to her. This is a very frustrating phenomenon. People can go to where you last logged out, and your avatar is there...just unresponsive. And you can't get back in.
Linden Lab claimed to have fixed this problem, back before Christmas. But this week, it's resurfaced. There is a Grid Status report on it here: http://status.secondlifegrid.net/2012/01/28/post1553/
If the suggestion there (logging into a different region than the one you left) does not work, there are a couple of other things you can try.
1. Log in with a different account (an alt). Pay your ghosted account $1L. This is often enough to break the log jam and complete the failed log out. If you don't have an alt, contact a friend who can get in world and try the same trick.
2. Contact Support. If you are a Premium member, you can use Live Chat. Otherwise, you will have to submit a support ticket. Links to both can be found here:
There has also been a JIRA (a bug report) filed on it. You can see that here: