The macbook pro does not turn on after a 6-month break.

Hi,

I suppose your MacBook Pro battery may be completely dry out. You should take your MacBook Pro to your local retail AppleStore, Genius Bar or Apple Authorized Service Provider to be examined, especially battery there

You would need replace the battery.

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I agree with @kaz-k's assessment. You can try letting the laptop charge for a day to see if it will charge the battery, but most likely it won't help. If a Lithium Battery is left completely discharged for an extended time, the battery will become permanently damaged. Whenever storing a Lithium battery (or a device containing a Lithium battery), the battery should be stored with a charge between 20% - 60% which means at some point the battery (or device) should be reconnected to power to make sure the battery charge level remains in that zone.

I have no idea how quickly the battery will drain if the laptop is completely powered off. You would need to spot check it after a couple of weeks to see how much it has drained to help you calculate a safe & optimal period between refreshing the charge level.