i didnt get any error in apex class code but when i tried to execute it shows some error like DMLhelper variable doesnt exist

DMLhelper.createBulkprospects();

In the code you are not adding lead to the list so leads were not creating. Updated the code as below.

public  class DMLhelper
{
    public  static void createBulkprospects()
    {
        List<Lead>lstleads=new List<Lead>();
        for(integer counter=1;counter<=100;counter++)
        {
            Lead ld = new Lead();
            ld.firstname='Bulk';
            ld.LastName='Lead record'+counter;
            ld.Company='salesforce INC';
            ld.Status='Warm';
            ld.Industry='Technology';
            ld.phone='9900998877';
            ld.fax='9900778899';
            ld.Email='bulked'+counter+'@gmail.com';
            ld.LeadSource='Web';
            lstleads.add(ld);
        }
        if(!lstleads.isempty())
            Insert lstleads;
        system.debug('number of DML statements used..:'+Limits.getDmlStatements());
    }
    }