Did you notice anything wrong before you walked away from the computer? Did you ever save this as an Audacity project or did you leave it open without saving it?
If you left it open without saving it, see https://forum.audacityteam.org/t/waveform-silenced-when-stop-recording-solved/39535/5 as a possible explanation.
If you did save it, did this problem happen when you reopened the project after File > Close or File > Exit?
If there is a saved AUP project file, please attach it. Attach the contents of Help > Show Log… whether you saved a project or not.
Gale