I've been using sonar 8.5 for about 3 years with a yamaha AN1X as a controller. I had to buy a new computer recently and I installed sonar onto it. When I plugged the keyboard in (using a midi to usb cable) it didn't work. The little keyboard icon doesn't light up, and in midi devices the only thing that comes up apart from the comptuer's sound card is the name of the cable, not the synth, and it only comes up in output not input. I don't know what to do cos I don't know what the problem is. Can anybody help?

You say you have a new computer and installed sonar. Though I am not familiar with the Yamaha ANX1, I suspect that you will need to install its midi drivers onto your new computer.

The MIDI/USB interface ("cable" as you call it) is what will show up in Sonar's MIDI devices list in preferences unless the synth connects directly via USB (i.e. it has a USB socket).
 
I assume all worked OK in your previous setup? If so then it should work now, in theory at least.
 
If the interface has drivers, you may need to install them.
 
With the interface connected if you go into Windows/Control Panel/Devices then select "view by connection" off the view menu you should see the interface hanging off a USB port in there if Windows is recognising it properly.
 
If it is then download a copy of MIDIoX if you haven't already got it and see what MIDI ports it picks up with the interface connected.