I'm running PiHole on a completely vanilla Raspberry Pi 3, with its Raspbian OS. It's currently set as my router's only DNS server.
The web dashboard was working just fine at pi.hole/admin, and then I upgraded from... a very old version, to whatever is most-current, and now neither that address nor 192.168.0.2/admin/ are responding to browser queries.
Any suggestions, aside from "starting over again"?
Pi-hole v6 uses a completely different web server, embedded into pihole-FTL.
When you upgraded, the previous web server was still running, so Pi-hole v6 installer changed the web interface port to 8080. Also, Pi-hole v6 offers https, on port 443:
[✓] tcp:0.0.0.0:8080 is in use by pihole-FTL
[✓] tcp:0.0.0.0:443 is in use by pihole-FTL