Reply from support (at) fr24.com:
Unfortunately Flightradar24 on Windows has been experiencing some technical issues, which meant the apps weren't available over the last 48 hours. We've now resolved the problems and the app should be working as normal, however if you're still experiencing an issue can I kindly ask you to restart your device and try the app again.
If you've chosen to uninstall the app, I'm sorry to inform you that due to very small portion of Windows App users, Flightradar24 was forced to make a difficult decision to stop providing the app through the Windows App store. If you've previously purchased the app you may be able to download and install it still by going to the Store --> My Library --> Apps --> Show all and selecting Flightradar24.
You can also try this solution if you still cannot find it:
To see what apps and games you've previously installed on your phone, go to Store Store Tile --> More More icon --> My apps.
http://www.windowsphone.com/en-us/how-to/wp8/apps/delete-or-reinstall-apps
Please also note that we're currently working on making our website more mobile friendly, which will help users view Flightradar24 across multiple mobile platform in the future. If you have more questions regarding he Windows App please contact the Windows App store.
If I've misunderstood your inquiry or you have any further questions or concerns, please don't hesitate to contact us again.
Kind regards,
Gaelan
Flightradar24.com Support