I had similar problem, suddenly I could not connect to our VPN server on OS X server at home. No matter if i was connecting in- or outside of our home network, the result was same = no connection.
Server's VPN service logs did not show even a failed attempt. Totally silent. Only when I changed to l2tp+pptp on server and changed clients profile to pptp i saw the failed login on VPN service's logs as i should.
For me the solution was to change the l2tp shared secret on server and of course on the clients too (iPhones, iPads)
Now it works again. Weird. Maybe some update caused a certain shared key to fail the l2tp service totally?