Doesn’t turn on

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Hi

Make, Model and OS could help.

Have you tried a Hard Reset?

https://www.dell.com/support/kbdoc/en-uk/000139016/how-to-perform-hard-reset

Although 3 days without power may have drained the main battery, and I would suggest (if there is no warranty issue and you feel capable) of disconnecting the main battery and any CMOS coin cell battery. 

Apply power.

Post any Pictures of the screen that you can please.