Sounds like it's trying to boot off the network instead of the hard drive.

First, at the ok prompt (you get there by holding the Stop key and hitting 'a')

Use the printenv command to see how it's set up right now - note what the boot-device is.
Try the following boot command
ok boot disk

You might have to change your default boot-device if it's set to just net

Check out these links:
Solaris - basics of using the boot command
Solaris - how to set an alternative boot