How to fix an "ImportError: No module named numpy" on AWS EC2 Server with AMI Linux but it installed?

For five days or so I could start a script with numpy without problems. I installed it like here in the forum proposed and it worked. when I try to start another python script with numpy today, I got the following error:

Traceback (most recent call last):
  File "neuralTensorNetworkAcc.py", line 8, in <module>
    import numpy as np
ImportError: No module named numpy

But numpy seems to be installed:

[ec2-user@ip-172-31-10-106 sk-learn]$ sudo yum install numpy                    Loaded plugins: priorities, update-motd, upgrade-helper
amzn-main/latest                                         | 2.1 kB     00:00
amzn-updates/latest                                      | 2.3 kB     00:00
Package python26-numpy-1.7.2-8.16.amzn1.x86_64 already installed and latest version
Nothing to do

What is wrong? How to fix that bug?

Thanks in advance!