Hi Robin, 

I am not able to boot into windows so I can´t make a Back Up

It's probably beyond the point of recovering Windows at this stage.  If the system shipped with Windows 7 or newer, you do have the option of installing a copy of Windows 10 for free at this point.

If there is data on the drive you need, you may be able to recover it by mounting the hard drive in an external case and attaching it to a working system.

The single worst part of the job is the disassembly required to change a drive on this model -- see:

< >

(as you can see, the system essentially requires a total strip-down to access the drive).

Reinstalling Windows 7 is a better option than win 10. The N laptop models do not run windows 10 well or at all due to lack of dell drivers. Borrowing a Dell reinstall disk of the same version of 7 or buying one on ebay if he doesn't have one should allow reinstalling without the win 7 key. But I would take this opportunity to replace the laptop instead. Lots of bargains at Dell today for Presidents Day.