General antonyms:
* Appear
* Materialize
* Come into view
* Become visible
* Show up
* Surface
More specific antonyms:
* Remain (if it's about staying in a place)
* Stay (if it's about not leaving)
* Persist (if it's about continuing to exist)
* Re-emerge (if it's about reappearing after being hidden)
* Endure (if it's about lasting through time)
The best antonym will depend on the specific situation in which you're using the word.