If your feed has been working normally and all of a sudden, without any changes made by you, it stops working, it is best to make sure the dongle is still working.
Here is how you can check if the DVB-T dongle you are using is working properly. It is always best if you connect the dongle directly to your Pi instead of via a USB extension cable.
1. Install rtl-sdr package. Run all the commands you see below one by one
sudo apt-get update sudo apt-get install rtl-sdr
2. Stop fr24feed as well as dump1090
sudo systemctl stop fr24feed sudo systemctl stop dump1090-mutability sudo systemctl stop dump1090-fa
3. Run the test command
If the dongle is OK, you will see an output like this
Found 1 device(s): 0: Realtek, RTL2832UFA, SN: 00001001 Using device 0: Generic RTL2832U Found Rafael Micro R820T tuner Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6 Sampling at 2048000 S/s. No E4000 tuner found, aborting.
(If you see a different tuner than Rafael Micro R820T/R820T2/R828D then your dongle most probably does not support ADS-B frequency.)
4. Reboot your Pi to start normal operation