I upgraded rb750 from ros 5.18 to ros v6 rc3. This was fine. Then I upgraded routerboard from 2.39 to 3. Routerboard rb750 didnt started, I had to power it off and pover it on. Maybe the cause of freeze after start was watchdog.
But propably routerboard.
What do you think?
Try NetInstall, see if you can access the router with that and reinstall v6 or try v5.