Not sure which version of QBDT you're using, but this should work...

To to File > Send Company File > Accountant's Copy > Client Activities > Remove Restrictions

It may be a little different with your version, but basically you want to go to where you created the Accountant's Copy and find something that says Remove or something similar. 

Second option if you can't find a way to Remove the lock.  If you have a copy of what you sent the Accountant, go through the same File menu and Accept the changes.  Since it's the file you created, there are no changes.  It should just release the lock.  But the first method would be the preferred choice.