My Blog Has Vanished

My blog at https://4petsonline.com was working just fine when I last viewed it yesterday. This morning I logged into my Admin and tried to delete a number of spam posts. I immediately received the error message as follows:
“The e-mail could not be sent.
Possible reason: your host may have disabled the mail() function…”
I logged out and when I tried to log in again I received exactly the same message and it also appears when I just try to view the blog, typing in its URL.

What the heck has happened?