For the majority of Avalons, you can restart the operating system in the head unit by pressing and holding the on/off knob (that's the same knob that serves as the volume knob). My deceased 2013 Avalon and my 2018 Avalon both reset in this manner. They both use a different head unit than yours. (You do have the factory unit, don't you?)
Another method that may work for resetting the head unit is to disconnect the battery for a few minutes (5-10 minutes?). While the battery is disconnected, press various buttons on the head unit to help dissipate any latent energy stored in the unit's circuit boards. You will possibly lose some customizations (presets, contact list, saved locations etc.) in your head unit after doing this. Additionally, the car may drive funny for 20-30 miles because the ECU may need to relearn your driving style.
These are general things to try. They is not a guarantee that they will work, but the cost to do them is $0. I was not able to locate a copy on Toyota's owners site that was specifically for the HDD Nav unit. I'd suggest going through the settings menu and looking for a reset option. If you find one, do it. Also, if you have a copy of the HDD Nav unit owner's manual, read it. It may offer a suggestion.