Can't boot after installation (debian5 on macmini)

[Ben Jakb <ben.jakb@gmail.com>]

hi
I've just installed debian 5 for the second time on a macmini. It
still doesn't boot, but only shows a blinking folder with an
exclamation mark.

Its a simple debian-only installation, no dualboot. 2 partitions: for
root and swap. GRUB was installed.

Any ideas?

thanks in advance
ben