Looks like your listener is not working properly. Check the listener.log file for details.
$ oerr ora 12500
12500, 00000, "TNS:listener failed to start a dedicated server process"
// *Cause: The process of starting up a dedicated server process failed.
// The executable could not be found or the environment may be set up
// *Action: Turn on tracing at the ADMIN level and reexecute the operation.
// Verify that the ORACLE Server executable is present and has execute
// permissions enabled. Ensure that the ORACLE environment is specified
// correctly in LISTENER.ORA. The Oracle Protocol Adapter that is being
// called may not be installed on the local hard drive. Please check that
// the correct Protocol Adapter are successfully linked.
// If error persists, contact Oracle Customer Support.