Grab app creates weird "masking" in Mc OS Catalina

When I try to use the Grab app in Mac OS Catalina, instead of the usual commands prompting to choose between full screen, selection etc. there is a motionless, lighter frame (as if Grab had selected a portion of the screen randomly) with everything around it greyed out and dark, as if the frame created a "mask" dimming the screen around it. Is there any way to reinstall the application or to correct the problem? Thank you in advance for your help.