XPS13 9350 dead overnight

In the evening my XPS13 9350 was still functioning OK, in the morning however, it did not woke up from the sleep mode anymore. The LED on the start button turns on for a few seconds, but nothing else happens. Any other LED is turned on, no noise from the body housing whatsoever /fan, disk,.../. Screen does not turn on either. Battery is "fully" charged (indicating by LEDs), charger works as well (charging LED on the front side is turned on, when plugged in). In the last month I noticed the battery charging always stopped at the 75% (the same problem accrued when laptop was brand new, but the battery was exchanged under warranty and problem disappeared until month ago).

Based on several suggestions found here, I tried to detach battery and drain the power, but without success. I have just ordered external disk housing to (hopefully) access to my data. 

Any other suggestions? Laptop is 2.5 years old and therefore not covered by warranty. In case motherboard died, I can simply throw away this 1400 EUR worth pieces of ***, right? Ohhh, go to , never buying it again.

BR, Jernej