This could easily happen if you create a proxy service against the sample echo service which ships with WSO2 ESB - and WSDL was pointed with an URI - and the corresponding WSDL is unavailable at the time proxy service been deployed [i.e. When the ESB is restarted]
To recover from this error - you need to edit the file [ESB_HOME]/repository/conf/synapse-config/proxy-services/your_service_name.xml and add the following parameter..
<proxy name="test" transports="https http" startOnLoad="true" trace="disable"> .............. .................. <parameter name="enablePublishWSDLSafeMode">true</parameter> </proxy>
Now restart the ESB and you will notice that this particular proxy service is not loaded - but the ESB starts fine.
To handle this issue in a more generic way - a fail-safe mode to be introduced in to the upcoming release of WSO2 ESB - further details here...