LCD shows only block characters  

LCD shows only block characters

I've been using my fully assembled Prusa I3 MK3 kit for about a month. I received the kit around the first week of Jun. Everything was great, and I am very pleased with the printer.

But yesterday, all of a sudden, while I was moving the Z axis up from OctoPi to get some clearance to clean the nozzle, the LCD went blank and only show two rows of block characters. (When this happened, the hotend is about halfway on the Z axis)

Everything still seem to be operational. I can still print fro OctoPi. But as the LCD is not operational, I cannot do any printer control using the LCD, like unload/load filament, calibration, etc.

I have tried to power cycle the printer. reseat the LCD cables, etc. I've also flash the printer with the latest firmware (version 3.3.0), but none of these fix the LCD problem. I also tried a different set of cables I took from my TEVO -- no difference.

Consider that I only get the kit for less than a month, should I request a replacement LCD? Or anyone has suggestion how I can trouble shoot this problem?

Thanks
Quan