Retroactive validation is sometimes an acceptable process...particularly in the case of legacy products that were originally documented under lesser requirements than we would understand now to be appropriate, but I've also heard of it being done in an instance such as yours.
You could perform a retroactive validation ASAP, both to resolve the documentation gap and also as a mechanism for establishing that the product hasn't been dangerous during the time between first marketing and the completion of that retroactive validation.
Properly, you should halt the use of the product while that retroactive validation is done. That of course might not be practical unless the number of units in the field is relatively small and you have contact information for all of them.
If the number of distributed units is large and/or you otherwise don't have effective control, and/or you don't have the ability to do the rretroactive validation in a hurry, you might not have any (proper) choice other than to call your FDA regional office's Recall Coordinator and discuss the situation with them.
In any case, I'd develop that validation plan as quickly as possible and get going. The FDA might regard a combination of forthrightness and urgency in fixing the gap as sufficient to justify waiting on a field action, if you can convince them that the risk is low.
