trigger SendEmailtoContact on Account (after update) {
    set<String> lstConEmail=new set<String>();
   
        List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
        Set<Id> setAccountIds = new Set<Id>();
        List<String> sendTo = new List<String>();
         If(Trigger.IsAfter && Trigger.IsUpdate){
        For(Account Acc : trigger.new){
            If(Acc.Active__c == 'Yes' && Trigger.oldMap.get(Acc.Id).Active__c != Acc.Active__c){
              setAccountIds.Add(Acc.Id);
            }
        }
    }

        if(!setAccountIds.isEmpty()) {
            for(Contact c : [SELECT lastname,Email FROM Contact WHERE AccountId IN:setAccountIds]){
                if(string.IsNotBlank(c.Email)){
                    lstConEmail.Add(C.Email);
                }
            }
        }
    sendTo.addAll(lstConEmail);

        if(!sendTo.isEmpty()){
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            mail.setSenderDisplayName('Email Alert');

            mail.setSubject('Account Name change');
            String body = 'Dear Employee Account Name is change.';
            mail.setToAddresses(sendTo);
            mail.setHtmlBody(body);
            mails.add(mail);
            try{
                Messaging.SendEmail(mails);
            }
            catch(Exception e){
                System.debug('-----Exception------' +e);        
            }
        }
}