First step I'd take is to check 'log -ct' via the SSH or Telnet CLI.
It should show the attempts coming in and would display if there is a username / password error (unlikely in your case) or something. If you see nothing, eg looks like no attempts are being noticed then a possible cause is PPTP service being disabled. Check that via the Web UI under VPN and Remote Access Control and if for some reason it's now unchecked you'll need to check it and reboot the unit.
It's also worth checking PPP General setup to see what's set in there. Mutual Auth should be disabled, and I'd expect the appropriate start IP to be set (typically 192.168.1.200 etc). If the router has DHCP enable then it'll allocate from the DHCP pool instead of the start IP I believe, but if DHCP is disable on the router it'll take the IP from the start IP.
If you're using RADUS or LDAP for auth then try a local account instead as a test.
Also check the WAN IP is the same as before, you're probably on static, but its worth checking anyway
