(I know I'm repeating what Thomasps said in his post, but...)
Yet now the Oracle Enterprise Manager hangs when trying to connect. It seems the database is not accepting connections?
This is rather a "broad brush" statement to make. Check that the database will accept connections by trying to log in using SQL Plus. If you can then the problem is with EM, in which case stop & restart the EM service.

Only if you can't log in using SQL Plus can you say that the database is not accepting connections, in which case a reboot of the server may help to put things right.