Run Dell ePSA Preboot Diagnostic to check the hardware within the system by following steps in link below. The Dell utility will rule out any faulty hardware that might be causing problems on the computer. Please report back any error code.

ePSA Pre-boot System Assessment (Official Dell Tech Support):

https://youtu.be/qeHNSxW7VSM

After that I can't turn of the laptop, with holding the powerbutton for 5 - 10 seconds so I have to wait till the battery is drained or disconnect the battery again.

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I suspect your computer is freezing up. This could be cause by faulty hardware such as hard drive, memory (ram) or bad battery. It also could be faulty windows updates, drivers or corrupt files. How old is the computer? windows 10 is on the computer? 

System locks up, freezes or an error occurs.

https://www.dell.com/support/article/us/en/04/sln129591/system-locks-up-freezes-or-an-error-occurs?lang=en

Your power button is simply broken . Two little plastic hands hold this button and they often break . The good solution is to change the full palmrest . The cheap solution is to glue the two hands holding the button with an epoxy glue