Turning LED On/Off by using SIM808 with Arduino Mega
Firstly your code was really poorly formatted, making it difficult to see what it did.

After reformatting my best guess as to why you LCD is blank is that the SIM808 is does not have any data available.  print something in the else statement.

Next it could be that the LCD is not working, print something out in setup().

You might also want to add a delay to loop() to prevent the Arduino spamming your SIM808.