virtualbox.org

I've lost all network connections on all VBox guests. I had them yesterday, and there are no problems on the host. I haven't made any changes in the vbox settings, or on the host that I can think of. I'm using a PCnet-FAST III adapter in each case. Does anybody know what might be the cause of this?

No idea. It's a needle in a haystack right now. You don't give enough information to even remotely think of a cause. Give full settings of the VMs, network configurations on the Host and Guests and then we can see what might be the problem. Also keep firewalls and system updates in mind.

The settings on the guest machines vary, but they are pretty much the defaults for their systems. As I said in the original post, I am using FAST III/NAT. The same thing happened on ALL of the guests, and no changes have been made on the host AFAIK: No changes in the network or firewall settings, no updates. Please be specific as to what information would be helpful.

What kind of connection to you have on the host?
Have you tried to shut down all guests and then shutdown the host (not reboot do a complete power cycle)?

I have a DSL connection. I'll try a cold reboot, but it will be a little while before I can do that.

With your DSL connection, do you have a permanent connection or do you have to create it first? If you boot the VMs before your Host has internet, the Guests won't have internet with NAT. They have to be shut down then started again (rebooting the Guest OS won't work, VB won't check the network that way).