Like Roman Snitko's response, I would say that clicking Reset and requeue all to move all files from the Failed tab to the Queued tab is the best way to go. However, please make sure you export your failed files list before you select Reset and requeue all.
Occasionally, when I select Reset and requeue all, FileZilla will display a message that says "Not all items can be requeued" and then CLEARS YOUR FAILED ITEMS LIST without entering them onto the queue, leaving you with no way to recover the list (I think).
This happened to me with FileZilla 3.45.1 and I have not yet determined how to recover the failed items list if this happens and no backup has already been created. I'm not sure if this is still a bug in later versions.