That depends on how it was deleted.
Posts can be deleted by several users of sufficient reputation (and consequently undeleted, if deleted in error). Posts can also be deleted by diamond moderators, who respond to flags by the community to remove posts that do not meet our faq guidelines.
I'd suggest your best bet is to have a look at the FAQ and see if there's anything in there that looks like it might be the cause, i.e.:
- Link only answers.
- Product advertising.
- Duplicating other answers.
- Old questions.
- Not answering the question (it may have been converted to a comment).
And so on. If you still feel having looked through all of those it was in error, you can raise a post here on meta. I would suggest if you have a link to the question or answer you're referring to, you edit your post and we can then explain :)