Why does temperature gauge no longer work immediately after replacing the spark plugs on a 1989 Chevy Caprice 305 engine?

Those two parts are not directly related, but they are just a few inches from each other. You could have disturbed to wire for the switch while changing the spark plugs. The switch is located between #1 and #3 spark plugs and a couple of inches up. Check the wire to make sure it is functional, even pull it off and put it back on. If it is not the wire, then you can try replacing the unit. It's around $15 at a local parts store. Hope this helps! Please let me know how you made out. sincerely, John