So, you have an SSH key pair. The public key has been added on to the server. On the client, ie your laptop/desktop you have your private key. For some reason, your private key has not been loaded into memory.
All you have to do is start the ssh-agent and then load the key into memory via ssh-add.
Step 1: Start the agent
eval $(ssh-agent) Step 2: load the key into memoryread more