I am running a Amazon ec2 instance on amazon's linux.

Whenever I run anything sudo yum it give me this:

sudo: yum: command not found

ec2-user$ rpm -qf /usr/bin/yum
yum-3.2.29-30.24.amzn1.noarch

ec2-user$ which yum
/usr/bin/yum

which yum while in root gives:

root$ which yum
/usr/bin/which: no yum in (/sbin:/bin:/usr/sbin:/usr/bn:/usr/local/bin:/opt/aws/bin)

This is a new ec2 instance two days old. When I first logged in I ran sudo yum update and everything wen't well. What changed?