Thanks for the reply Chrispy. Yes we only need TNSlistener and OracleService service to remain functional but in my case not just the agent is down, it has also taken the DataGatherer and most importantly the TNSlistener.
I can no longer connect to Oracle from either Svrmgrl.exe or SQL Plus. As TNS is down,I get Protocol adapter error.

Prior to this agent service problem, all services were happily running (DataGatherer, HTTPServer, TNSListener, OracleService), now they just follow their Agent :( .