I was recently testing endstops on my RAMPS board when I saw I had one plugged in backwards. I turned off the power and fixed the endstop, but when I turned on the power supply the LCD was completely white with all the pixels on. I can still see the LCD working if I view it from an angle and it works fine if only powered by only USB.
Can you measure the 5V output ( eg. on the endstop headers) if it's still 5V or more?
You have to power from RAMPS not USB and better remove all other stuff on RAMPS.
You might have killed the onboard 5V regulator and now it's regulating at a higher voltage. Hence the superbright LCD backlight.
Yup, the onboard regulator is toast
You could also use an external 5V psu ( cellphone charger ), but that requires some modifications.
I haven't done it myself, so I'm not the person to give you advice.