Q: my 3rd generation apple tv will not come on just a white flashing light my first generation one is working fine? tried unplugging it nothing, any help is much appreciated.

You probably need to restore it:

Apple TV (2nd and 3rd generation): Restoring your Apple ...

Click on the blue link I gave.  You would need to hook it to a computer running iTunes using a microUSB lead.

If you have an Apple Store nearby take it there and see if they can sort it out.

It is possible the display resolution is wrong - you could try holding menu and up arrow on remote together for 6 seconds which will make it cycle display resolutions - but only if it is actually working but showing a black screen due to wrong resolution selected.