GSM/GPRS module is not responding in AT command

Goodevening to all!

I just starting my project on controlling bulbs and outlets using relays via SMS sending codes when i encounter this problem. In serial monitor, when I put the command AT, the serial monitor didn't respond. What can be the problem/s or how can I make it work?

I am using arduino uno and gps/gprs shield sim 900d. I'am using adapter with an input of of 9V ,300mA I haven't used the module since last year and it contacted with dust. but all status, network led and power led is working properly.

Thank you for your answers!