Have a Focusrite Scarlett interface that's been fine. Today I set up a new desktop, and so on hooked stuff but not the power cord to the interface. Usb was not connecting so a few time plugged and unplugged it and the power to see if that would help the computer find it. But now the power light and the phantom light rapidly blink and never steady!! I tried a completely different socket in another room and same thing. Any ideas?? Maybe he power cord?? I don't have another like it to try...
 
Thanks

What model Scarlett do you have?  Some will work either with the USB port providing power, or with an external power supply.  Could also be a problem in the new PC.  Have you tried a different USB port on the PC, and if plugging into a front USB port, plug into a rear or vice versa.  
 
There have also been issues with some units and a USB 3.0 port.  My backup recording interface unit is a 6i6 2nd Gen and it works OK with a USB 3.0 port.  However, something to consider as many new PC's only have USB 3.0 ports.

This 8i6 is powered by a a cord  and not the USB. So in this case I am sure the usb not connecting is because the power is screwed up. I hooked up another interface...Presonus Audio box 1818VSL and it works fine on new computer.

I wrote to Focusrite for help ...will see what they think soon.
 

I'd look at the USB connection first, since the only thing that's changed is your PC.  Try a different port (USB 2, not USB 3), or try another USB cable if you have one.

Are you saying the lights flash even with the USB cable disconnected? I would try the unit as a stand alone and see what happens. Disconnect the USB or simply don't turn on your computer. Plug a mike and some headphones in. You should hear the mike. 
It sound more like the power supply. 
I just looked and my 6i6 uses a 12 Volt 125 ma  += centre pin so not hard to replace. 
 
FYI everyone, only the Solo, 2i2 and 2i4 use USB power, they don't have the option for a a power supply which sucks. All other models use power supplies. 

The 8i6 uses the same adapter as the 6i6, so shouldn't be hard to replace.  A quick search, and I found a few on both Amazon and Ebay for around $12-15 USD.
 
Still, IMO the most likely culprit is the USB3 port since that's the problem 90% of the time when switching to a new system and everything else is the same. 
 
To the OP, you should be plugging the USB cable into a USB2 port (black inner strip), not a USB3 port (blue inner strip).  This needs to be ruled out before you invest in a new power supply.

DIDD try a different port but not different usb cord...will do and report back

I wish so but the different usb cord did not fix this. So back to power cord problem to find out . As cactus music suggested I have not yet tried it as a stand alone. Never dawned on me that it could be ...as a test. Until now I missed that post. I hooked up my Presonus Sound Box 1818, usb and power cord...it works fine

Well it's a pretty old unit as you can tell by looking for information. All I found on Focusrite web site about flashing power lights is for new units and saying make sure to connect your power supply which makes no sense as how are you going to have a light come on  all? At no point do you mention installing the drivers( Mix control) to you new machine.