EC2 instance with "instance store" cannot be stopped. They can only be terminated.
If you would like the ability to stop an instance, I suggest recreating the instance with an EBS root device. These types of instances support the ec2-stop-instances command. You can consider this command as a "pause", since it can be resumed at any time, and you will not be charged for the time the instance is stopped.
Amazon announced the ability to boot instances from an EBS root volume only in December 2009, so you will find that older documentation and tutorials ignore the extended command-set that came with the EBS root volumes.
Further reading: Amazon EC2 Instances Now Can Boot from Amazon EBS
I am not the owner of the Amazon
account and I won't be able to contact
with him for weeks.
If you really want to "pause" this instance for a few weeks, you can create a machine image (AMI) of your instance and terminate it.
You will then be able to launch a new instance with your private AMI when you want to "resume" this instance. It will not be the same instance, as in it will have a different instance ID and a different IP, but you will be cloning the setup of your instance.
The methods to build an AMI differ if your instance is running Windows or Linux, but you should be able to find adequate information on the web about both scenarios.