This is one of the very often faced error for job failure.
Exact error goes as follows:
Unable to start execution of step (reason: Error authenticating proxy , system error: Logon failure: unknown user name or bad password.). The step failed.
This happens when the secret word(password) you provided while creating a credential for a proxy has got expired. Exact scenario may be, you have used a domain account which enforces password expiration policy and you have changed you password. This happens because sql server resolves your changed password when you authentication mode is set to accept mixed mode, but the credentials which you have created does not accept this or resolve the change. Now you if you have to make this proxy work you need to delete the credential and again create it with new password and map to proxy. This sometime may again hit your jobs. So better way is to alter the credential and provide the new password. This can easily be achieved by below script by replacing the values according as per your needs:
ALTER CREDENTIAL [ProxyCredentialsName] WITH IDENTITY = N’DOMAIN\ACCOUNTNAME’, SECRET = N’NewPasswordchanged’