John-pero is correct. We're unable to revert a voided entry, but you may recreate it using the history in your QBO account.
To find voided transactions, use the Audit Log. This report will show you the history or detail of a modified or deleted entry. Here's to use it:
Click the Gear icon, then select Audit Log under Tools.
Use the Filter button then choose Show only these events.
Mark the Transactions box to show all the actions made to a certain entry.
If you only want to find deleted entries, pick Deleted/Voided Transactions from the Show drop-down.
From there, locate the deleted transaction and press View.
Click on Show all to get the details needed to recreate the entry.
If there are deleted items, recreate them if necessary by clicking on the + New button on the toolbar. Here's an article to guide you on how to utilize the Audit Log: Use the audit log in QuickBooks Online.