You can re-open a matter in 2 places:
Matters screen
Within DPSOneoffice, click on File to drop the menu down.
Click on Matters.
Enter the full matter ref into the field named Matter Ref.
Click Search.
Double-click on the entry for the matter you need.
Once the next window opens, click on the Completions tab.
Untick the Close Matter field and click on Save.
The matter will now be open.
Ledger Card
From the Ledger card for the matter, click on the Completion tab.
Un-check the Close Matter tick box.