i entered this in url

https://c.ap1.visual.force.com/apex/sendemail?i=0019000000ONI8K

Controller

public class sendEmail {
public String subject { get; set; }
public String body { get; set; }

private final Account account;

// Create a constructor that populates the Account object
public sendEmail() {

account = [SELECT Name, (SELECT Contact.Name, Contact.Email FROM Account.Contacts)
from Account where id = :ApexPages.currentPage().getParameters().get('id')];
}

public Account getAccount() {
return account;
}

public PageReference send() {
// Define the email
Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();

String addresses;
if (account.Contacts[0].Email != null)
{
addresses = account.Contacts[0].Email;
// Loop through the whole list of contacts and their emails
for (Integer i = 1; i < account.Contacts.size(); i++)
{
if (account.Contacts[i].Email != null)
{
addresses += ':' + account.Contacts[i].Email;
}
}
}

String[] toAddresses = addresses.split(':', 0);

// Sets the paramaters of the email
email.setSubject( subject );
email.setToAddresses( toAddresses );
email.setPlainTextBody( body );

// Sends the email
Messaging.SendEmailResult [] r =
Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});

return null;
}
}

vf page is 

<apex:page controller="sendEmail">
<apex:messages />
<apex:pageBlock title="Send an Email to Your
{!account.name} Representatives">
<p>Fill out the fields below to test how you might send an email to a user.</p>
<br />
<apex:dataTable value="{!account.Contacts}" var="contact" border="1">
<apex:column >
<apex:facet name="header">Name</apex:facet>
{!contact.Name}
</apex:column>
<apex:column >
<apex:facet name="header">Email</apex:facet>
{!contact.Email}
</apex:column>
</apex:dataTable>

<apex:form >
<br /><br />
<apex:outputLabel value="Subject" for="Subject"/>:<br />
<apex:inputText value="{!subject}" id="Subject" maxlength="80"/>
<br /><br />
<apex:outputLabel value="Body" for="Body"/>:<br />
<apex:inputTextarea value="{!body}" id="Body" rows="10" cols="80"/>
<br /><br /><br />
<apex:commandButton value="Send Email" action="{!send}" />
</apex:form>
</apex:pageBlock>
</apex:page>

Error:System.QueryException: List has no rows for assignment to SObject 

System.QueryException: List has no rows for assignment to SObject