I am running VirtualBox guest instances of Linux on a Mac OS X host. Sometimes when I open the laptop lid I find that the guest instances are unable to connect to the network--it simply times out!
The best solution I've found so far is to disable and enable the network interface in the guest, by right clicking on the little network icon on its window, and selecting "Connect Network Adapter..." consecutively (first to 'disconnect', then to 'reconnect' again). Then the network comes back to normal.
I'm guessing that perhaps the VirtualBox daemon or whatever it uses in Mac gets killed when the host is suspended, but I've no idea of how to restart it (and no will to dig it out). Other solutions could be totally closing and starting VirtualBox again, or even your laptop! But they are way more obtrusive, and this is faster. Like a gentle 'turn off and on again' ;-)