If the Search bar still works, search for powershell. Then right-click Windows PowerShell in the search results and select Run as administrator.
But if the search bar doesn’t work either, press Winkey-R, type powershell, and press Enter. Right-click the PowerShell icon on the taskbar and select Pin to taskbar. Close the PowerShell program. Hold down Shift and Cntrl while you right-click the PowerShell icon again. Select Run as administrator.
Once you’re in the Powershell command-line environment, type the command sfc /scannow and press Enter. Then wait. It can take quite a while.
Eventually, the program will tell you either that it found no corrupt Windows files, it found and fixed them, or it found some but couldn’t fix them. If you get the latter response, try the command
dism /online /cleanup-image /restorehealth
This should (hopefully) download replacement files.