I was making the quest from the College of Winterhold, where I was being used to test some spells from an Elf (I don't remember her name), and then she transformed me in a horse, and then she put me back, but I was not there!

I thought that it was a secondary effect from the spell, then I continued playing. A few minutes later, I realized that it was a bug, but it was too late, all my auto-saves were overwrited and I cannot go back.

I updated the game, but it is still missing. When I use a Invisibility Potion, it appears to be there (with the invisibility form), but when it goes away, it dissapears again.

Is there some way to go back and make the dragonborn to appear?

BTW, sorry for my bad english.

From the UESP:
The player may remain invisible after the last phase of the quest (needed for the simulation of being transformed into an animal).

• Use the Become Ethereal shout to fix it.

I think this applies to your situation.