The todoist icon has disappeared from my google chrome What can I do
If you have by mistaken hidden the icon you can undo it :). Click on the 3 lines at the top right, go to Tools, Extensions. From there you should be able to find Todoist. There should be a button called "Show Button" which will re-add it to the bar.
If Todoist does not show up here, then it was somehow removed and will need to be reinstalled from the Chrome Store Page.