Media play/pause & track prev/next stopped working after reboot

Do the following:

gsettings set org.gnome.settings-daemon.plugins.media-keys previous ''
gsettings set org.gnome.settings-daemon.plugins.media-keys next ''
gsettings set org.gnome.settings-daemon.plugins.media-keys volume-mute ''

and then try again as the keys won't be trapped any more so Gnome can do whatever it wants with them.

If one of the keys stops working, rebind it in Keyboard settings.

Here's my solution on Ubuntu 19.10 with media keys working after a clean reboot, but not after resuming from sleep, etc.

Restarting gsd-media-keys in Terminal:

killall gsd-media-keys
/usr/lib/gnome-settings-daemon/gsd-media-keys

You might want to run the latter command with the run dialog (Alt + F2) because it will start in the foreground.