At the time of writing, the Flightradar24 website displays a “canceled” message and it is not possible to access the service.  Flightradar24 is a popular tracker to monitor flights in real-time, including commercial trips, freight, repatriation flights, and, now, potentially military aircraft.  On Thursday, as Russian forces invaded Ukraine, Flightradar24 has experienced traffic rates 10 - 20 times higher than normal. 
“Due to extremely heavy load, some users may experience slowness or temporary connection issues accessing Flightradar24,” the organization says. “We’re working on increasing available performance now.”
“We are still working hard on increasing the capacity, but the very big international interest generate[s] 10-20 times higher traffic than normal, which is hard to handle.”
Civilian flights in Ukraine have been restricted and the country’s airspace is closed due to military action. Ukraine cited a high risk to safety and the European Union Aviation Safety Agency (EASA) has now listed Ukraine’s airspace as an active conflict zone in a new Conflict Zones Information Bulletin (CZIB).  “Under these circumstances, the aforementioned airspace and critical infrastructure, including airports, are exposed to military activities which result in safety risks for civil aircraft,” EASA says. “In particular, there is a risk of both intentional targeting and misidentification of civil aircraft.” Flightradar24 has been tracking what appears to be military craft, including a US Air Force Northrop Grumman RQ-4 Global Hawk. 
An alternative service that does not appear to be facing disruption is Planefinder.net.  Update 15.26GMT: Flightradar24 is still experiencing heavy traffic demands and is working on expanding its available resources.  Some users receive Cloudflare error messages while using VPN connections. Separately, it appears that some visit requests are triggering “sorry, you have been blocked” errors.  “Some users may have noticed a virtual waiting room,” the organization said in an update. “We’re using this as a temporary measure while we add resources. We hope to remove it shortly to make Flightradar24 instantly accessible for all users.”