For S4, if the miner powers up, the back light of the small LCD comes on but screen shows blank, highly likely it is the MicroSD card either got loose inside, or data on the microSD card got corrupted.

You will find the MicroSD card inside the case, behind the LCD panel and next to the PSU.  There is a Black Colored Beagle Bone Black and there is a little MicroSD card slot on it.  Lightly push the card and it will pop right out and push it back in.

Then please try to power it back on and see if it boots up.  If not, you may have to use this software to reimage (Write) the factory default data of S4.

Image Software (Windows):
https://bitmaintech.com/files/download/SD-partition-tool.zip

S4 Factory Default Image:
https://bitmaintech.com/files/download/initramfs.bin.SD-20141016.tar.gz

Any issues, please PM or contact our live support at 844-248-6246 Option 2

Thank you!