Kindly help. Installing on a virtual machine

Ok so Im installing Ubuntu on a virtual machine on widows 7 (I have to use a virtual machine for my assignment). I followed all the steps necessary to install but each time I open the virtual machine they request that I install Ubuntu again. Any idea why?

Thank you for your help.

Did you remove the installation media (iso/cd) from the VM?

Yes I did. And it worked. Thank you so much.