If you accidentally unmatch a user, you can see them in your unmatched & blocked list. To match with a user again, you must upgrade to premium first, as re-matching is a premium feature. Then in the unmatched and blocked tab, you should see the option to match again next to the user's name. Simply click this button, and the user will be matched with you again, and you will find them in your matches tab.
*** Please note, you can only re-match a user once. You will not have the option to match again after unmatching.