If a restore of eM Client doesn’t work as @skybat advised above, it appears you need to create new encryption keys as they are normally “not recoverable” as per post below from the following thread.
@Charlie6
Dec '18
It is not possible. All you can do is create a new key pair and send out or publish that public key ASAP. It wont decrypt previous messages sent to you, so those will be lost unless you have them resent with the new public key.
If you have published your public key to a key server, you can use the revocation certificate (if the key was generated outside of em client) to remove it from the server