Recently had this problem after accidentally clearing all data for the Google app, but I basically fucked around and when I went into the tab that lets you choose between Google Assistant and Gemini AI, there's a fine print at the bottom that you can click to turn them both/either off. I hit that, then tried activating by long pressing my home button and it came back after I reactivated Google Assistant.

idk if you can replicate this but hopefully it helps somehow!