Have you been through al the issues discussed here, and the usual stuff too?
Providing sufficient power to the SIM800 (that can draw a couple of amps)
Tx to Rx and Rx to Tx
Communicating at the correct baud rate
If you’re using SoftwareSerial on an Arduino then not exceeding 9600 baud
Not using the same serial port for debugging and SiM800 comms
This might help you to understand some of those issues better…