First power cycle your soundbar or AVR, and make sure it is on the correct input (whatever input your Fire TV is plugged into). This is usually the issue and varies by soundbar and AVR, so is hard for us to to help with remotely.
The Fire TV remote is programmed through the interface on your Fire TV. If you are having problems with your soundbar not turning on, or the remote not controlling the volume of the soundbar properly, you may need to re-connect the Fire TV remote to the soundbar.
To do this, from the Home screen, go to go to Settings > Equipment Control > Manage Equipment > Add Equipment > Soundbar
From there, follow the instructions which vary depending on how your soundbar is connected. You may need to manually power-on the Soundbar for this to work.
You may also have to adjust the the Power Timing, which is at Settings > Equipment Control > Manage Equipment > Advanced Settings > Power Timing. Try increasing this setting.
Hopefully that fixes it, but if not, and your soundbar is connected to your TV with an HDMI cable, the problem could be related to the CEC settings, which control how the devices communicate with each other (your TV, your Fire TV, and the Soundbar). These settings are both on your TV and on your Fire TV (and possibly the Soundbar).
The Fire TV CEC settings can be adjusted under Settings > Display & Sounds > HDMI CEC Device Control. Make sure this is on. Your TV CEC settings will vary depending on your TV manufacturer. Contact your TV manufacturer if you are unsure how to adjust these settings. Your soundbar may also have some settings that you can adjust related to power control. You should check with your soundbar manufacturer for more information.
All of these steps vary depending on how your system is connected... whether the soundbar is connected to the TV via an optical cable, or an HDMI cable, and if the Fire TV stick is connected to the TV or directly to the soundbar. If everything was working, and suddenly stopped, the above steps should get things back to normal.