It all depends on what "removing" means for you. If you really remove it via the Workflows in the List settings, than association is gone. This means you need to go again in Workflows and re-associate it. Then it depends on how it has been created: via SPD or custom code, or e.g. Nintex/K2, etc. - also depends whether it was a List workflow, Reusable (which means it could be Content-Type attached) or Site.
Try opening up your site via SPD and search for it starting with your list, site, on the left-navigation panel. If you're lucky, maybe you only removed association, which means that you could redo it, otherwise you might not be happy about deleting it.
If you simply forbid it to "new instance" (2nd option out of the 3) you re-instate it easily, again via List Settings - Workflows.