They will cancel it, and then you will cast your ballot like a traditional vote
PLAN YOUR VOTE. PLAN ON IT TAKING HOURS.
Can't take ballots to the polling places.
Won't be able to vote on election day by a regular ballot if absentee was requested.
Every state has their own laws.
Info should be available on your state's Secretary of State website.
This kind of thing has been asked and answered dozens, maybe hundreds of times in the past two months.
As several others have already said, check the laws in your state. Every state is a bit different. Some absolutely require mail in only, so if your ballot isn't already in the mail, you may be s.o.l. on your ballot being received and counted. Unless your state accepts and counts ballots received after November 3.
The internet is your friend.