Unfortunately there's not currently any history/revision system in place, though I know it's one of the things the staff want to implement in the future. In the mean time I have to strongly recommend that you keep a backup copy of any submissions that you make to the site, so that you can resbumit in the unfortunate case that something like this happens (which thankfully is relatively uncommon).
Content plagiarised from other sites is absolutely not tolerated here. Can you please send me a private message with details on which articles have the content copied from wikipedia? Thanks!