Help -error msg -sim900 gsm shield and connection with arduino uno

[ArduinoNoobNeedHelp]

So I have conncted my Arduino UNO to a Simcom SIM900 GSM Shield and I am using a TMOBILE SIM CARD. My Arduino is connected by USB and my GSM by 5V 2.5A adaptor. My code is correct but when I open up the serial monitor I get the error: DB NO RESP ERROR: SIM900 Doesnt Answer.Check power and serial pins in GSM.cpp

If you could please help me, I would greatly appreciate it! II need this for an important project!!!

I am just trying to get a text sent and this is what my code looks like

#include "SIM900.h"
#include <SoftwareSerial.h>
#include "sms.h"
SMSGSM sms;

int numdata;
boolean started=false;
char smsbuffer[160];
char n[20];

char sms_position;
char phone_number[20];
char sms_text[100];
int i;

void setup()
{
    Serial.begin(9600);
    Serial.println("Serial Started.");
    
    if (gsm.begin(9600)) 
    {
        Serial.println("\nstatus=READY");
        started=true;
    } 
    else 
        Serial.println("\nstatus=IDLE");

    if(started) 
    {
        if (sms.SendSMS("+19569758590", "Arduino SMS"))
        {
          Serial.println("\nSMS sent OK.");
        }
        else
        {
          Serial.println("\nError sending SMS.");
        }      
    }
};

void loop()
{
    if(started) 
    {
        sms_position=sms.IsSMSPresent(SMS_UNREAD);
        if (sms_position) 
        {
            Serial.print("SMS postion:");
            Serial.println(sms_position,DEC);
            sms.GetSMS(sms_position, phone_number,20, sms_text, 100);
            Serial.println(phone_number);
            Serial.println(sms_text);
        }      
        delay(2000);
    }
};