amangue
There are many possible reasons, you really need to tell us more details about your network setup etc.
Did external web access to your server ever work from the start, or did it NEVER work ?
Try doing a open port scan at grc.com from a workstation behind your server.
Is your server in gateway server mode or server only mode ?
Is your sme server behind a standalone router ? If so have you forwarded port 80 from your router to the sme server ?
Check various log files using the View log files panel in server manager eg the http access log files, also messages log file.
Are your external DNS records configured correctly for your domain and point to your external IP (assuming you have a static IP), or otherwise is your dynamic DNS setup correctly (for dynamic external IP).
Please tell us your real domain name so we can test your server externally.
Please answer all questions.