When you type commands, text jumps around on the screen and nothing works the way it’s supposed to.

Make sure you’re not typing the J command when you mean j.

You may have hit the CAPS LOCK key without noticing it. vi is case-sensitive; that is, uppercase commands (I, A, J, etc.) are different from lowercase commands (i, a, j), so all your commands are being interpreted not as lowercase but as uppercase commands. Press the CAPS LOCK key again to return to lowercase, press ESC to ensure that you are in command mode, then type either U to restore the last line changed or u to undo the last command. You’ll probably also have to do some additional editing to fully restore the garbled part of your file.