Of those I have seen, serial port goes in “disable” while core is in application running mode.
Can It be enabled ?
I’m not what you mean by disabled but you can turn on the COM port with Serial.begin (9600); in setup ()
More information here:
What I mean is that , when working local,
(building with etc Netbeans and connecting to local server),
core is at DFU mode in order to upload firmware,
and after uploading, core restarts and serial communication does not exists.
So even if serial.begin(); has been used, no serial is availiable.
Might need to see your code and maybe provide us with more information?
It will work but probably just some settings issue. 
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I use pytty as a serial monitor on windows, and then it’s important to close the putty session before the spark resets.
When you don’t get a serial connection:
- close any existing serial monitors
- hit reset on the spark core
- wait until you see the breathing cyan led
- Then connect to serial.
On Windows you can find the correct serial port by using Device Manager - it will be listed under “Ports (COM and LPT)”.
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As soon as core runs new uploaded firmware, serial connection is not working , not even serial driver is present at control panel. ( I am working local using Netbeans and local server).
Please post your code and also list as much detail as you can - e.g. which OS/version are you using?
If it’s Windows, then try removing/reconnecting the usb cable to the core. Failing that, a reboot - I’ve seen drivers become permanently disabled in Windows, only available again after a reboot. Also be sure to close all programs that might be trying to access the serial port.