Suddenly, POs for one of the suppliers have been marked as "Closed" without being fully paid. It's not possible to change back to "Open" to add bills against. Any idea?

I know a way to add bills against Purchase Orders, hrmeili.

Once the Purchase Order status is Closed, we cannot attach the bill. We'll have to delete the transactions, then recreate them. 
 

Here's how to delete the purchase order:

1. Go to the Search icon on the top menu.
2. Choose Advanced Search.
3. Click the Search drop-down button and choose Purchase Orders.
4. select search, then choose the purchase order to open it.
5. Click More at the bottom page of the purchase order.
6. Choose Delete.
7. Then, click the Yes button to confirm.

Then, recreate the transactions.

1. Go to + New.
2. Select Purchase order.
3. From the Supplier ▼ dropdown, select the supplier.
4. Enter the necessary details.
5. When you're done, select Save and close. 

Once done, add purchase orders to bills.

Additionally, you can run purchase order reports in QuickBooks Online. This helps you track the transactions and ensure they're all recorded accurately.

I'd suggest getting back to this thread if you need help managing your purchase orders. We're always here to assist you.