Have you checked the logs aif-WebApp.log and odiagent.log for errors.
Bet you would have tried SUNOPSIS (default password), if not give it a shot.
Do follow the article - FDMEE Tutorial: How to Change the ODI SUPERVISOR Password.
If the above solution didn't work next stop is this - https://odielt.wordpress.com/2017/03/01/how-to-supervisor-password-in-odi/
When going directly to the URL that the ODI Agent listens on though (http://server:port/oraclediagent - if default ports are used then something like http://localhost:6550/oraclediagent) then the agent does appear to be up and running (as it returns a 'ping' response). Check if this is working. If not, review the odiagent.log file for more information.