Check if it needs a software clear command. This can reset the screen sometimes.
I'm not sure if the links i posted below will work as its seems it was working fine but during printing somehow broke.
Let me know if it doesn't and we can dig a little deeper. What electronics are you using? What firmware and Arduino IDE do you have? Was any of the 2 recently updated?
I am using Sanguinololu board, Marlin firmware, Aduino IDE 1.0.1. They are not updated. I just re-uploaded the firmware, but that firmware did work before, but I don't know why it not working now.
You could try depowering and repowering it.
Like most of these displays, there is no physical reset of the onboard chips, and if they get confused, they loose the plot.
After re-uploading the firmware for some times, it works again now!. I think maybe its is communication issue that cause the firmware was not completely upload to mainboard.
I am using Geeetech Prusa I3B, with LCD 2004 smart controller, and the printer has been working perfectly for a couple of months. Yesterday, when I was printing, the LCD suddenly showed only 2 lines of blocks!
At first, I thought it was just constrast issue, but after I adjusted it, the screen still not worked, I checked the connection of LCD for many times, and I can confirm the connection is fine. I can not find any other solution. Can somebody help me? Thank you in advance.