It can be tempting to reset a deck that hasn't been reviewed in a while, but I do not recommend doing so if a large number of cards have graduated and have a review history. Despite feeling like you may have forgotten everything, you likely remember more than you realize. Instead, I suggest reviewing the overdue cards.
If you select the deck and review the overdue cards,
the cards you still remember will increase their intervals significantly,
while the forgotten cards will not lose their intervals completely,
as long as you have new interval set to a value greater than
In the end, this approach will require you to review fewer cards than if you started from scratch.
If you reset the deck,
you'll have to relearn each card again,
with each card graduating with an interval of
1 day (or the value you set in settings).
You'll lose any progress you have made on cards that you still remember.
If you find yourself struggling with a backlog of cards, consider reading Fighting backlogs. The most important tip is to store overdue cards in a filtered deck and take small chunks for review. With this approach, you avoid overwhelming yourself with hundreds or thousands of cards at once.