Hi,
There are a few things here that could be the cause.
1. Postgres is not running
2. If Postgres is on another server, can the EM server connect to that other server? Run a telnet from the EM server to the Postgres server to ensure communication is ok. Also check if a firewall is blocking it.
3. Check your <EM Home>/config/tess-db-cfg.xml and check if the parameters are correct, especially the url.
4. Put your EM in DEBUG and restart. Once you see that an attempt to connect to the db has happened, upload your log here and we can give it a quick look.
Thanks,
Matt