I had 500 tabs and they suddenly randomly disappeared

For some reason I was going through a pdf on Safari, once I closed it I noticed that the 490 tabs I had were basically all deleted and the only 15 most recent ones remained. What do I have to do to get those tabs back? I tried seeing if they were closed but they don’t appear, it seems as if they’re lost somehow, but I really wish to recover them. All help is appreciated, thank you.