VF:
<apex:page controller="AccountWrapper" sidebar="false">
<apex:form >
<apex:pageBlock >
            <apex:pageBlockButtons >
            <apex:commandButton value="Select" action="{!isSelected}" rerender="table"/>
            </apex:pageBlockButtons>
            <apex:pageBlockSection title="Accounts">
            <apex:pageBlockTable value="{!wrapItems}" var="c" id="table">
                <apex:column headerValue="Select">
                   <apex:inputCheckbox value="{!c.selected}"/>
                </apex:column>
                <apex:column value="{!c.accList.Name}" headerValue="Account Name"/>
            </apex:pageBlockTable>
            </apex:pageBlockSection>
            <!--<apex:pageblockSection title="Related Contacts">
            <apex:pageBlockTable value="{!accContacts}" var="c1" id="table">
            <apex:column value="{!c1.Name}" headerValue="Contact Name"/>
            </apex:pageBlockTable>
            </apex:pageblockSection>-->
</apex:pageBlock>
</apex:form>
</apex:page>

Controller:
public class AccountWrapper 
{

    public List<WrapperAccounts> wrapItems{set;get;}
    public List<WrapperAccounts> selectedItem{set;get;}
    //public List<Contact> accContacts{set;get;}  

    public AccountWrapper()
    {
        wrapItems=new List<WrapperAccounts>();
        List<Account> accItem2=new List<Account>([SELECT Name FROM Account limit 10]);
       
        for(Account a1:accItem2)
        {    
            boolean check=false;
            wrapItems.add(new WrapperAccounts(check,a1));

        }
    }
//Method to Fetch Records.
public PageReference isSelected()
    {
        selectedItem=new List<WrapperAccounts>();

        for(WrapperAccounts w:wrapItems)
        {
            if(w.selected==true)
            {
                
                fetchRecords();
                
            }
            PageReference p=Page.WrapperReturn;
           return  p;
       }
        
        
        return null;
        
    }
//Method to define fetchRecords
public void fetchRecords()
    {
            List<Contact> accContacts=new List<Contact>(); 
  
     for(WrapperAccounts wraps : wrapItems)
     {  
      for(Contact con:[Select Name from Contact WHERE AccountId!=null])
      {
       if(wraps.selected)
       {  
         accContacts.add(con);  
       }  
     }  
     System.debug('Contacts Lists:'+accContacts);
    } 
    }   
//Wrapper Class
public class WrapperAccounts
    {
        public boolean selected{set;get;}
        public Account accList{set;get;}
        
        public WrapperAccounts(Boolean sel,Account acc)
        {
            this.accList=acc;
            this.selected=sel;         
        }
    }
}

I am not getting the Contacts related to the Selected  Accounts. Button is not working. Even if I give setter getter method accContacts then also it's giving the error that dereference to null.