If your private key is lost or destroyed, but not compromised, you can no longer sign or decrypt messages, but anything previously signed with the lost key is still valid. This can happen, for example, if you forget the password used to access your key, or if the disk on which the key is stored is damaged. You need to choose a new key right away, to minimize the number of messages people send you encrypted under your old key, messages which you can no longer read.