Voiding the Sales Order will set the Quantity of the Items on the Sales Order to zero thus setting the amount to zero as well. The Closed checkbox on the item line is also marked thus the status will be set to Closed. Furthermore, there would be a memo on the Memo field that says "VOID".
To undo the voiding of the sales order, follow the steps below:
1. Edit the Sales Order.
2. Enter a Quantity for the Items on the Sales Order.
Since the Rate was not erased when the Sales Order was voided, it will automatically compute the Amount based on the Quantity entered.
3. Unmark the Closed column on the item line.
4. Remove "VOID" message on Memo field.
5. Save the Sales Order.
"Warning: This Transaction is voided and will remain voided even if you make modifications to it. Are you sure you want to modify it?"
When you get the message, just click on OK button to save the Sales Order.
Once you save the Sales Order the status will go back to Pending Fulfillment and will be ready for processing again.
Right now, there is no way to completely remove the message. This is still an Enhancement Request (Issue 29114 - Create a feature to allow voided transactions to be un-voided at a later date).