"SWD adapter not detected" error message

I'm getting a "SWD adapter not detected" error message when I try to connect, what could be wrong?

On connection, CrossStudio checks the presence of the SWD adapter by putting the adapter into SWD mode and cycling the SWDIO pin. This error message is output if the state of the SWDIO pin read back does not match the state being set.

This error message could be caused by the following:

• No SWD adapter attached - check that you have an active SWD adapter connected (http://sites.fastspring.com/rowley/product/armswdadapter for example) and not just a simple 20-pin to 10-pin cable converter.
• Target board is not powered - the SWD adapter requires a voltage reference on the board's VCC pin in order to work.
• Device being held in reset - check that the target device is not in reset.