It's down for me. Getting this message on the login page: "ProtonMail cannot be reached right now, please try again later."
On the log-in page I'm getting:
503 Service Unavailable
No server is available to handle this request.
I had the login page cached in my browser, so there's a bit of javascript that pops up an alert.
They suffered a major DDOS attack a while back, and I'm guessing that their servers being down intermittently is going to be part of their normal operations.