I have sparcstation 20 and when I am starting it, in the begining of boot process it shows me:
(Can't load specfs)
and throws me into ok prompt.
What can I do?

This looks like a nvram issue which had to solve by reinstalling the OS (Solaris) and specifying the boot disk down to the slice.

Older systems translate target 3 to disk0. If you type devalias at
the ok prompt and the rev of boot prom is high enough you should get a list of disk aliases. You may have done that. Sounds like the disk just moved to a funky state. Sometimes on the older systems probe did not work right unless you did power on.