Post excerpt from alert_XE.log file that shows the database has been started & is now up & online
 
ora-12514 ALWAYS only occurs due to a problem on DB Server system.
One cause of this problem is when the Oracle database is down & needs to be started.
A remote client send a request to the Listener asking to be connected to a specific service.
If/when the listener does not know anything about that service, the listener responds with ora-12514

post results from the following two OS commands:
lsnrctl status
lsnrctl service

Since every connection request to the listener gets logged, listener.log file will contain a line with 12514 as status code.

This line contains valuable debugging details. So post this line & surrounding lines.

for additional debugging suggestions read the URL below:
http://edstevensdba.wordpress.com/2011/03/19/ora-12514/