If you are using Qubes-Whonix, complete the following steps.
Qubes App Launcher (blue/grey "Q") ->
Whonix-Gateway ProxyVM (commonly named 'sys-whonix') ->
If you are using a graphical Whonix-Gateway, complete the following steps.
Start Menu ->
If you are using a terminal-only Whonix-Gateway, press on Expand on the right.
Complete the following steps.
sudo service tor@default Restart
Check Tor's daemon status.
sudo service tor@default status
It should include a a message saying.
Active: active (running) since ...
In case of issues, try the following debugging steps.
Check Tor's config.
sudo -u debian-tor tor --verify-config
The output should be similar to the following.
Sep 17 17:40:41.416 [notice] Read configuration file "/etc/tor/torrc".
Configuration was valid