I did something similar and fed the nano 12v on one of it's inputs which killed it.
Plug a mini-USB cable into it and see if you can communicate with it via the Arduino IDE. The Serial monitor ought to work, or you should be able to upload a new sketch.?
As John says, if the red led is flashing then the bootloader is running so it should be responsive to being reprogrammed. There's also a little reset button on the nano you can try and there's a procedure for resetting it via pins.
If the LED is steady and you get no response then it's probably dead. I had this and had to replace it. They're cheap enough to buy from eBay but it's quite a lot of desoldering and resoldering to get the little blighter off and put the new one on. I also damaged one of the tracks in the process and had to bypass it with a piece of wire but it's all working again now.
