What errors did you get when you tried to access EM12c?
Have you checked if the OMS is up? If not, can you start it?
In case, you need to locate OMS log and trace files, see this doc:
By " default Enterprise Manager installed with Oracle Database: you mean the db console that is configured in the DB. With out removing the dbconsole the EM 12.1.0.x installer will not let you proceed on the interview screens itself. So how did you finish the EM install?
Another suggesting that i can give you is to use EM DB template for setting up your DB to have EM repository. These EM db template are available on OTN for 126.96.36.199 /3 EM and while using these templates the dbca gives you can option to "not configure Db console" that will save you time later on while doing the EM install.