Accidentally pressing and holding the light bar for a bit too long will sometimes result in your Wave resetting and returning to the weakest default temperature level.
To use Single Press to return to your last used settings, instead of pressing and holding, make sure to click once, briefly, on the red or blue side of the light bar to initiate a warming or cooling session.
If your Wave has run out of battery since you last used it, it will forget your last used temperature level. If that happens, you can readjust by using the - / + buttons in the app or by pressing and holding the relevant side of the light bar.
To avoid this, make sure to charge your Wave when you notice it start to flash amber to indicate a low battery level.