Old photos disappeared

Was out of the country for a while, returned to my computer and opened Photos (MacBook Air running High Sierra 10.13.3). All of my photos from a previous old Mac are gone. Held down Option while opening Photos and two libraries appear, one of which has ALL the old photos. Tried to export to an external hard-drive, wouldn't work. How do I migrate them into the current version of Photos? It includes all my albums, or "Events". Thank you!