I am working with db2 V9.1 on AIX 5.3.I am trying to restart the instance.I am getting the
error:- db2start : SQL8000N DB2START processing failed; a valid product license was not found. If
you have licensed this product, ensure the license key is properly registered. You can register the
license via the License Center or db2licm command line utility. The license key can be obtained
from your licensed product CD.

>db2 db2licm -l will show installed licenses.
If you don't have a valid license installed then you'll need to obtain a license file (a .lic file) and install it using the
>db2licm -a <license file>

If you don't know where your .lic file is, then you should contact IBM

it might be that the lic file was installed after the instance was created
execute db2iupdt from root (see manuel for correct syntax) to reflect the lic to the instance