|You mean to say; CD/DVD or USB disk is not detected at machine start-up?|
If you cannot start the installation of the OS from within Win 8 try the following:
Most likely Win 8 is installed with UEFI interface of your machine. This means you will not see the POST (classic BIOS) but immediately boots into windows 8.
Changing the start-up (or BIOS) settings; open PC settings > Update and Recovery > Advanced startup > click restart now. After a few seconds A new screen appears:
Either you can Reset your PC (like a factory reset) and install the new OS or with Advanced options start a command prompt and open de CD/DVD/USB from there.