If a product shows as completed, no further work can be recorded against it. However, you can change it's status back to in-progress to resume work.
Production Overview Report
Open the Production Overview Report and change to List View
Scroll down to the production order that needs to be changed from completed to in progress.
Double click on the order line, a window will then open. Change the Status to Suspended.
Load Works Orders
If the product was loaded as a works order, you will also need to change the release status in the Load Works Orders page.
Find the order, click on it and the lock icon, then change the status to Not Released.
Try running a production sequence now, the order should be released as in progress.