Sim900/Uno Project not Connecting to Network

Hello, I'm trying to make phone calls using a sim900 shield with an uno. It worked before but suddenly its not working now. The board powers on and I see a red light which remains solid without blinking. And I cannot connect through it with a call

On the serial monitor

DB:NO RESP
DB:NO RESP
DB:NO RESP
Trying to force the baud-rate to 9600
ERROR: SIM900 doesn't answer. Check power and serial pins in GSM.cpp

Here is the code I am using.

https://pastebin.com/vr1CCNH8

How can I start testing the sim900/uno to get to the bottom of what is wrong?

BTW I uploaded a blank sketch and typed AT into the Serial Monitor and saw nothing at a 4800 baud rate.