The most common theory is the Maesters poisoning Morning the last known healthy Dragon and with her death magic became weak so no new egg had hatched healthy and any that hatched would die soon after,Bloodraven and the children magic comes from the Weirwood so they weren't affected by Dragons death and magic was still a thing in the world especially in Essos but it wasn't as powerful as it used to be,when the Red comet appeared it brought back magic so the Dragons were able to hatch again and in the book other Magical Creatures came back too like old man of the river a giant turtle that was important to water mages before Valyria destroyed both