ssh-add returns with: "Error connecting to agent: No such file or directory"

You need to initialize the agent first.

You can do this in multiple ways. Either by starting a new shell

ssh-agent bash

or by evaluating the variables returned by ssh-agent in your current shell.

eval "$(ssh-agent)"

I suggest using the second method, because you keep all your history and variables.

The SSH agent is not running, or the environment variables that it sets are not available in the current environment (most importantly SSH_AUTH_SOCK), or they are set incorrectly (pointing to a dead agent).

You could start the agent in the current shell session using

eval "$(ssh-agent)"

or start a new shell session through the agent using

ssh-agent fish

(replace fish with whatever shell you are using). But since you say that you used to be able to use ssh-add without this, it leads me to believe that you've accidentally killed the agent (or it has terminated due to some other reason). The error message makes me think that the SSH_AUTH_SOCK environment variable is actually set, but that ssh-add can't find a valid communication socket at that path.

It would not surprise me if your usual way of doing things would work again if you completely logged out and logged in again, or rebooted the machine.

In Windows PowerShell (run as admin):

1. Check the current status of ssh-agent:

   Get-Service | ?{$_.Name -like '*ssh-agent*'} | select -Property Name, StartType, Status

2. Enable the Service if it is disabled:

   Set-Service -Name ssh-agent -StartupType Manual

3. Start the Service:

   Start-Service ssh-agent

4. Add your key as before:

   ssh-add <path to the key>