Not sure what FlightRadar24 is running as it's web server but most likely, lighttpd, a requirement of Pi-hole, took over and replaced that.
Which is weird, because it should have failed at starting the service, since there should have been something there, serving the FlightRadar24 data, on port 80.
Do you know what webserver the FlightRadar24 is based on ?
L.E.
Upon further analysis, it looks like the FlightRadar24 runs a custom app (which I assume is part web server) called fr24feed.
I have no idea how that is configured or even if it allows you to define the web port.
You best bet now is to move Pi-hole's lighttpd port from the standard 80 to something else, say 8080.
Edit your /etc/lighttpd/lighttpd.conf file and change server.port = 80 to server.port = 8080 or to whatever value you want within the standard port range (make sure the port IS available).
Then, you can run this:
sudo systemctl restart lighttpd.service
and
sudo systemctl restart fr24feed
That should fix it.
You Pi-hole would be available at
ip-address-of-rpi:8080/admin
Or, once the lighttpd changes have been made, reboot your raspberry pi.
One thing to keep in mind, any Pi-hole updates, will overwrite the lighttpd.conf file and you will have to do this again.