GitHub automatically deletes inactive SSH keys to help keep accounts safe, such as after someone leaves a job or loses a computer.
You can check if you haven't used an SSH key in a year by reviewing your account's security log. For more information, see "Reviewing your security log."
After your inactive SSH key is deleted, you must generate a new SSH key and associate it with your account. For more information, see "Generating a new SSH key and adding it to the ssh-agent" and "Adding a new SSH key to your GitHub account."