Hi,
1. Recheck your hardware connections properly, take care that no data lines are getting sorted.
2. Try varying the variable resistor.
3. check whether you are following below steps to intialise LCD in the code
Command –> RS=0, RW=0, E=1
Data –> RS=1, RW=0, E=1
4. To Intilaise LCD in 8-Bit mode use command 0x38 in hex or 56 in decimal format.