@Medhya,
Please look and help me to figure it out.

public class SetToWonController {
    
    public Id OpportunityId;
    public Opportunity OppDetail { get; set; }
    public List<SelectOption> contactRoles { get; set; }
    public Contact_Role__c crc;
    public String selectedContactRole { get; set; }
    public Contact contactDetail    { get; set; }
    public List<Contact_Role__c> oppRoles { get; set; }
    public boolean crcAvailable { get; set; }
    
    public SetToWonController() {
        OpportunityId = ApexPages.currentPage().getParameters().get('OpportunityId'); 
        OppDetail = new Opportunity();
        contactDetail = new Contact();
        if( OpportunityId != null ){
            OppDetail = [ 
                SELECT Name, 
                       Client_Delivery_Cut_Off__c, 
                       Purchase_Order_Date__c, 
                       Accredo_date_required__c,
                       Has_Accredo_fields_been_updated__c,
                       StageName, 
                       Type 
                FROM   Opportunity 
                WHERE  Id = :OpportunityId 
            ];
            GetContactRoles();
        }
    }
    
    public PageReference save() {
        OppDetail.StageName = 'Project Management';
        UPDATE OppDetail;
        List<Contact_Role__c>crcList = new List<Contact_Role__c>();
        if(selectedContactRole != '' && selectedContactRole != 'null'){
            List<Contact_Role__c> crcOld = [ SELECT Id, Payment_Contact__c from Contact_Role__c where Payment_Contact__c = true AND Opportunity__c = :OppDetail.Id ];
            
            if(crcOld.size() > 0) { 
                if(crcOld[0].Id != selectedContactRole) {
                    crcOld[0].Payment_Contact__c = false;
                    crcList.add( crcOld[0] );
                }
            }
            Contact_Role__c crc = new Contact_Role__c();
            crc.Payment_Contact__c = true;
            crc.Id = selectedContactRole;
            crcList.add( crc );
            if(crcList.size() > 0){
                update crcList;
            }
            
            if(String.isNotBlank(contactDetail.Email)){
                Contact_Role__c cr = [ SELECT Contact__r.Id,Contact__r.Email from Contact_Role__c where id = :selectedContactRole ];
                contactDetail.Id = cr.Contact__r.Id;
                update contactDetail;
            }
        }
        
        List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
        Messaging.SingleEmailMessage mail        = new Messaging.SingleEmailMessage();
        Opp_Won_Email_Setting__c ows = Opp_Won_Email_Setting__c.getInstance( 'Won Email' );
        List<String> sendTo = new List<String>();
        sendTo.add(ows.Email__c);
        if(!Test.isRunningTest()){
            OrgWideEmailAddress owe = [SELECT ID,IsAllowAllProfiles,DisplayName,Address FROM OrgWideEmailAddress LIMIT 1];
            mail.setOrgWideEmailAddressId(owe.Id);
        }
        
        mail.setToAddresses( sendTo );
        mail.setSubject( 'Opportunity Won' );
        String body = 'The Opportunity ' + OppDetail.Name + ', ';
        body += '';
        body += 'stage is set to Project Management';
        mail.setHtmlBody( body );
        mails.add(mail);
        
        Messaging.sendEmail(mails);
        return null;   
    }
    
    public void closePopup(){
        Opportunity_Tab_Controller otc = new Opportunity_Tab_Controller();
        otc.closePopup();
    }
    
    public List<SelectOption> GetContactRoles() {
        
        oppRoles = new List<Contact_Role__c>();
        contactRoles = new List<SelectOption>();
        
        oppRoles = [ SELECT Id, Name, Contact__r.Name, Contact__r.Email, Contact__r.Id, Payment_Contact__c from Contact_Role__c where Opportunity__c = :OpportunityId ];
        
        if(oppRoles.size() == 0) {
            crcAvailable = false;
            ApexPages.addmessage(new ApexPages.message(ApexPages.severity.Error,'Contact Role Does not exists,Please add Contact Role and Try again'));
            return null;
        }
        else{
            crcAvailable = true;
        }
        
        for(Contact_Role__c cr : oppRoles){
            contactRoles.add(new SelectOption(cr.Id,cr.Contact__r.Name));
            //if(cr.Payment_Contact__c == true){
                selectedContactRole = cr.Id;
                contactDetail.Email = cr.Contact__r.Email;
            //}
        }
        return contactRoles;
    }
    
    public void CheckContactRoles(){ 
        Contact_Role__c conRoles = new Contact_Role__c();
        conRoles = [ SELECT Contact__r.Id, Contact__r.Email from Contact_Role__c where id = :selectedContactRole ];
        contactDetail.Email = conRoles.Contact__r.Email; 
    }
}

Thanks In Advance,

Pavan Vempati.