This is the OpenBoot PROM monitor prompt. From this prompt, you can boot the system (from disk, CD-ROM, or net), or you can use the go command to continue where you left off.
If you suddenly see this prompt, look at the messages above it to see if the system crashed. If no other messages appear, and you just typed Stop-A or plugged in a new keyboard, type go to continue. You might need to Refresh the window system from its Workspace Menu.
Never invoke sync from the prompt without first running the fsck(1M) command, especially if the file system has changed.