No need to create a new matter for closed matters. If you have archived a matter accidentally or just wish to re-open a case, you can just simply unarchive it.
1. On the Matters tab, click Filters
2. Select Archived to show the list of your archived matters
3. Tick the box beside the matter name you wish to unarchive, then click the Unarchive button (green icon with an arrow pointing upward)
Please note that archiving or unarchiving matters won't affect any financial reports.