Uploading Problem

I'm a novice, and have been successfully running some of the tutorial programs.
Today I tried to upload a fairly long program, and got an error:
"avrdude: stk500_recv(): programmer is not responding"
I've tried resetting with the re-set button, disconnecting the circuit, disconnecting USB and power, with no results. The onboard LED keeps blinking.

I tried to upload a very short program....same result.

It's a Diecemilla board...running on OS 10.5.5

Any ideas? Thanks in advance!
Bruce.