Did you delete it from the WordPress Dashboard Menu screen or did you delete it from theme/plugin code?
If from the Dashboard Menu screen, that means that you’ve deleted it from the database and to retrieve it you’d have to restore from a back-up if you have one. That’s really your only option other than re-creating it.
If you deleted it from code, whether a theme or a plugin, you’ll need to replace the files with any local copies you have stored somewhere. If you have don’t have the code saved anywhere then, like with the DB deletion you’ll need to restore from a back-up.
If you aren’t saving copies of your code somewhere and you aren’t backing up your site, then you’re going to have to recreate it. Also, look into something like http://managewp.com or install a plugin like “BackUp Buddy“.
I’d go with ManageWP but that’s entirely a personal preference.