Could be due to scheduled maintenance, OS update or other reason.. like a power outage. Or perhaps a problem at your local ISP center. Try at different times of day, preferably during "light" traffic hours.
Major ISP services seldom have a backup generator, only battery backup for when a power outage happens! And that may serve only 24 hours or so.
F@H.. to solve mankind's maladies.. in our lifetimes!