If you have a copy of your ssh keys (e.g., on a USB stick) then simply copy the key files to the ~/.ssh/ directory.
cp /path/to/my/key/id_rsa ~/.ssh/id_rsa
cp /path/to/my/key/id_rsa.pub ~/.ssh/id_rsa.pub
change permissions on file
sudo chmod 600 ~/.ssh/id_rsa
sudo chmod 600 ~/.ssh/id_rsa.pub