All the pdfs I had on Ibooks disappeared suddenly

I was opening the books app oneway and all of a sudden I can't find any of the pdfs I had on the app . these were pdfs I had originally on my Mac . those pdfs mainly and specifically are Uni books that I study from them, add notes, highlight text in using Mac . and I had been sending them to my iPhone and open them through books app so that I can get back to them when I don't have my Mac . but now all of a sudden they disappeared from both MacBook and the iPhone .

the books were already on the downloads or the software of the MacBook . idk till now how could they be deleted like that ?

like I understand if the things were all on the books app from the start and just only sent to another device , but how did this happen even though they are saved on the Mac itself outside the books app ?

like they literally disappeared totally from both devices .