Why can't I copy and paste on my MacBook Pro?

Hi: For the past couple of days I have been unable to copy and paste anything on my MacBook Pro (2015 era, Retina, 15-inch). At first I thought it was some sort of feature or block on a website where I was trying to paste a quote into a comment I was making. I couldn't paste it either by Control-v or 'paste' on the drop-down menu. But today I tried to copy a url and paste it into an email I was composing. No-go. So then as a test I tried copying and pasting my own text from an email into a new email (so no websites involved) and it would not paste. Ditto on photos. They won't copy and paste. So this is obviously something going on with the computer or keyboard (built-in) that it's not allowing me to copy and paste regardless of what it is or where it originated. Is this a setting maybe? I've had this computer a lot and copy and paste a lot and never an issue until now. I've tried both ways of copying -- Cntl c and 'copy' from drop-down menu, as well as both ways of pasting. Thank you.