You didn't forget the insulation layer on the shielding then? It certainly sounds like a short and it's easily done. If it has shorted then who knows what has happened.
Where did you send your Amiga? Was the box damaged or motherboard poorly packaged in the post? Did you connect anything else to it upon first test besides the floppy drive? The bracked of the floppy can short out too if it's not correctly screwed in place.
It sounds like I'm assuming you have shorted it out...honestly I'm not! But it is easy to make these mistakes! Even plugging a mouse in can short something out if you're not careful.
