You have another key binding that begins with Ctrl+A, something like this:
A sequence of multiple keystrokes pressed one after another (rather than simultaneously) is called a “chord”.
You might have accidentally created it, or might have recently installed an extension that adds the problematic binding. To find it, do like I have done in the screenshot and type “ctrl a” into the Keyboard Shortcuts search box (that tab can be opened by typing Ctrl+K then Ctrl+S). Then look for a binding that begins with Ctrl+A but has something else after it (in my case, another Ctrl+A, but it could be almost anything).
When you find the offending binding, right-click on it, and either remove or change it. Then Ctrl+A alone will resume working.
Note: This question and its answer pertain to Visual Studio Code, which is different from Visual Studio. See this question for information about the equivalent situation in Visual Studio.