As a security precaution, GitHub automatically deletes SSH keys that haven't been used in a year.
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 in your GitHub account.
After your inactive SSH key is deleted, you must generate a new SSH key and associate it with your account. For more information, see this guide.
You'll also be able to find out more about the security feature in GitHub's documentation.