Enter the BIOS and find “Boot device priority” or “boot order”. Make sure the disk you have your operating system installed on, is on that list, and is selected as the first device to boot from. If that looks fine, then you can try to force a refresh. Turn it off and on, and, when you see the rotating loading symbol, hold the power button down until either the computer shuts off, or you see a screen that says something like "Preparing Automatic Repair" . If it just shuts off, try again. You may need to try a few times.
When you get to the automatic repair screen, select “Advanced options” and then “Troubleshoot”
Then select “Refresh your PC” and follow the instructions from there