What firmware are you trying to use?
If you are using Hugen79's firmware then pay close attention to the image you downloaded. I ran into a black screen after upgrade as well caused by me mistakingly flashing my NanoVNA-H4 with the -H firmware. The H and H4 are different hardware and their firmware is incompatible. Furthermore check the back of your unit and make sure you've selected the right hardware variant of the H (MS, SI, ZK) not doing so will also result in a failure to boot.
Luckily recovering is easy, just boot into DFU mode and flash the right firmware for your H/H4. I hope that helps.
