restart the MacBook and hold down the OPTION key until you see, on the screen, a number of startup choices.
The HD should be one of them. Choose that and you should get back in.
Once you get back in, go directly to system preferences, Startup Disk and change the choice back to the internal HD.
You can only startup from a network if there is a valid startup image hosted out on the network.
On the subject of passwords. The OS must have told you a least a couple of times that having no password was NOT a good idea. OS X requires an Administrator to approve the installation of most software and by leaving that field blank, you are leaving your machine open and vulnerable.
Let us know how you get on.
I have MacBook Pro 2.4GHz Penryn 15". OS X Leopard, 6 months old.
In trying to add up to a wireless network at home (with a PC and a laptop Windows XP) I switched to "start from network" on System Preferences. On reboot, shows image of file with a ? or flashes a world clip. Usual command+S or C, Alt, attempt to reboot from CD fail. Before attempting anything else I don't want to lose my documents. When reboot with Alt, the padlock with space for password appears. I didn't set a password when I installed OS.