Sometimes it happens that the target host, where you are trying to connect to, was re-installed. In that case your attempt to connect may be refused by your client - simply the host fingerprint has changed and it’s fingerprint record in known_hosts doesn’t match the current one (your client then thinks it might be man-in-the-middle attack).
If you are sure that it’s OK to accept new fingerprint, the easiest way is:
ssh-keygen -R <hostname>
This will force update of known_hosts with new fingerprint of specified <hostname>.