If you mean that the Volume icon is also gone to, then this tutorial (Method One) may help you.
Notification Area - System Icons
If it is just the volume bar, then you might try restarting the computer, or logging off and on, to see if works afterwards.
Hope this helps,
I had a online chat session with HP. I had to go to programs, pc help and tools, recovery manager, advance option, hardware driver reinstallation and then pick my keyboard app. and click reinstall, then reboot and all is back to normal. Pass this along for all other HP systems users.
Thank you for posting back the solution you found and results.