My notifications stopped a few days ago too.
I looked at my current email account to try and make sure the notifications weren't being blocked by the spam filter. The advice on their FAQ was to add the address to the contacts list. So I did that but still no notifications.
Then I looked at my WP Account Settings and was surprised to find the email address there was that of a now-defunct account that I used to use a long time ago here. I changed it back to the one it needed to be, but still no notifications. Then I discovered that the Edit Profile page also has an email address, which was the defunct one, so I changed that to the correct one, but on submitting the change, it said "ERROR: This email is already registered, please choose another one."
So I'm rather baffled and don't know what else to try. Don't know if this is related, but a day or two ago I noticed my gender in Account Settings in the had mysteriously switched itself to "female." It wasn't affecting what it said about gender on my posts, but I set it right anyway.