A window with some text and a blinking cursor should appear. Type sfc /scannow and press enter.

SFC checks system files against presumed good ones cached on your drive. Verification progress is displayed as it works through all files.

If you are in Windows Recovery, use sfc /scannow /offbootdir=C: /offwindir=C:\Windows\ instead. Replace C with the letter of wour Windows drive. If you don't know, assume it is C.

Once complete, a brief readout of the results will be shown. If errors were found, status of the repair is also noted.