USB connection. The mini USB end is notorious for getting too loose. A poor HDMI will cause corruption in the image data. USB causes a CSDA, no signal, button flashing, I do not know what the CSDA stands for but, I believe it represents all the signals that are working. various types of damage and you won't get CSDA. This is assuming you had a working setup before the failure. Emulated games may not readily adapt to OLED buttons.
Edit added:
The small board can have component failure on it, usually detected by a simple visual inspection. Fairly common with short and spill damage.