While loop in test bench of and gate. getting no output
Your code never enters the while loop because count is initialized to zero before checking the while==100 condition. Even if the condition wasn't broken, the brace after it would break the rest of the code. I think this is what you're trying for: 

// Inputs
reg in1_t;
reg in2_t;

// Outputs
wire out_t;

// test variables
integer count;

// Instantiate the Unit Under Test (UUT)
and_gate and_gate_1 (in1_t,in2_t,out_t);

initial 
   begin
      // Initialize Inputs
      for (count=0; count&lt;=100; count=count+1) begin
         //case 0
         in1_t &lt;= 0;
         in2_t &lt;= 0;
         #1 $display ("out_t=%b",out_t);

         //case 1
         in1_t &lt;= 1;
         in2_t &lt;= 1;
         #1 $display ("out_t=%b",out_t);

         //case2
         in1_t &lt;= 0;
         in2_t &lt;= 1;
         #1 $display ("out_t=%b",out_t);

         //case3
         in1_t &lt;= 1;
         in2_t &lt;= 0;
         #1 $display ("out_t=%b",out_t);
      end
   end