Flashing question mark but can still start up

I do not turn off my iMac with Catalina but, just lately, when trying to use it after a few hours of inactivity, I sometimes find a white screen with a flashing question mark. Most information on the web indicates a problem with the start-up disc or OS. However, after powering down the iMac and pressing the start-up button, it starts up normally. Any suggestions as to what is wrong?