is the browser working when trying to access from the local machine (the one NzbDrone is installed on)?
or is this only happening when trying to load the page from another machine on the network if this case then you would need to add the default port (assuming you haven’t changed it) to the windows firewall
you could also try disabling the firewall and just seeing if that changes anything and whilst your at it if running as a service check that it is running if so try restarting
cannot say if any of these would help but perhaps things to try if you have not already let us know if these work or not and we can go from there and try and help you diagnose the problem