Is this the multi-lingual restart message on the translucent grey background with the large picture of the "Power On/Off" symbol? If so, that's a kernel panic. If it's happening regularly, you might want to think about running the Apple Hardware Tool that comes on your OS X restore discs. It will scan your hardware for any faults. Faulty RAM in particular can cause this type of behaviour. How do I know? I just had to replace a faulty 1GB SO-DIMM in my PowerBook last week that was causing the same symptoms.The title says it all, I don't know what to do.