If I already paid for the tickets and I missed the flight do I have to pay again knowing that I paid already

Depends on the airline. Some of them will offer you an alternate flight or allow you to change the ticket date. My son went to LA to help family, they got him to LAX to late, the left, but he was offered to get on the plane to Vegas and wait 6 hours for the next plane to ATL, which was better that the 1st offer of waiting 2 days. Frontier was the airline

Every airline have their regulation and you have to pay fine

There is a bunch of ifs: If you ticket allow to reschedule; If your company allow this; And so forth. You need to ask the company, not the airport