For me the problem was for VS Code 1.51.1 and GitLens 11 they have made the Source Control Layout as default and requires the selection of view in order to make it work as before i.e., in the VS Code sidebar (using Mac OS Catalina 10.15.2).
After update:
- Press cmd+shift+p (ctrl+shift+p on Windows) and type > Gitlens: Show Welcome View and hit enter. This will bring back the GitLens on sidebar. Then click on GitLens icon in sidebar and you will be shown a welcome page similar to screenshot below:

- Click on Welcome (Quick Setup), this will open the below screen:

- Scroll to the bottom and then select the view named GITLENS LAYOUT among the two options given:

- Now click on Close button in GitLens menu that is opened next to VS Code sidebar:

And your GitLens is back!