That's only one step:
1. Have it repaired by Dell; most likely a new motherboard and/or new SSD.
If Dell has to replace the SSD, they put Windows 10 on it again. If the SSD can stay, because they only replace the motherboard, it probably still has Windows on it, although it's possible they return it to factory conditions during the repair.
I hope you have a good backup of your private things. Not guarantee at all they still are on the laptop if it returns from Dell.
If the repair is out of warranty (I see this model is from May 2011), repair most likely is too expensive. See if it works again if you only replace the SDD and use your recovery media to install Windows on it.
Hello, I have a Dell XPS 15z laptop with Windows 10 and I no longer boots neither hard nor on a stick with Windows or Cd / dvd absolutely nothing
Gives me the error "System not found" and dupaceea
"Check cable connection
Existing Intel PXE ROM "
PS. I forgot to mention above, the hard drive is a 128GB SSD