Having Error in transmission of Serial commands to A6 GSM module

Serial communication happens only one time But next time the If loop executes then serial commands did not execute.
Code worked for 3 months but it stopped working on my machine suddenly please guide

char phone_no1="9860422730";

void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
delay(400);
pinMode(13, OUTPUT);

}

void loop() {
// put your main code here, to run repeatedly:

if(digitalRead(8) == HIGH)
{

Serial.begin(9600);
delay(500);
Serial.print("AT+CMGF=1\r\n");
delay(3000);
Serial.print("AT+CMGS="");
Serial.print(phone_no1);// first number
Serial.print(""\r\n");
delay(3000);
Serial.print("hi");

Serial.write(0x1A);
Serial.write(0x0D);
Serial.write(0x0A);

Serial.print("\n");
delay(3000);

digitalWrite(13, HIGH); // turn the LED on (HIGH is the voltage level)
delay(100); // wait for a second
digitalWrite(13, LOW); // turn the LED off by making the voltage LOW
delay(100);
digitalWrite(13, HIGH); // turn the LED on (HIGH is the voltage level)
delay(100); // wait for a second
digitalWrite(13, LOW); // turn the LED off by making the voltage LOW
delay(100);

}
digitalWrite(13, HIGH); // turn the LED on (HIGH is the voltage level)
delay(1000); // wait for a second
digitalWrite(13, LOW); // turn the LED off by making the voltage LOW
delay(1000);// wait for a second
}

Code worked for 3 months but it stopped working on my machine suddenly

Then it is nothing to do with your code and you have a hardware issue.
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Not likely the reason but why a "Serial.begin(9600);" in loop()? The one in Setup() ougth to be enough.