Do the above, or delete your cookies as Carpetted! told someone else in another thread. And if you didn't save changes to turning off invisible, do so. Or just log out and log back in, but uncheck the "Invisible" bar.
There's too many threads like this, methinks. Maybe a sticky or something for it. *Cough.*
And I don't see why so many people are having so much trouble with this.. I tried invisible mode once, but was able to get out of it. Unless I'm still invisible now.
Well, try switching your skin first. Then, turn off invisible mode. After that, if you get off invisible mode, just switch back to the skin you are in, and you'll get off invisible mode!