RouterOs 4.5 upgrade problem with RB411

RouterOS Beginner Basics
1

I have successfully upgraded routeros on 1 x RB411 from 3.1 to 4.5 and 1 x RB433 from 3.3 to 4.5

However, I have tried to do the same upgrade using the same process, i.e. via webbox admin, on 2xRB411 and both of them have now ceased to function! No sound on power on, just dead.

What could have gone wrong? I waited more than 10 minutes before power cycling.
Can anyone or MT please throw light on this weird problem?

2

Connect Null Modem cable to COM port of the RouterBOARD 411. What console output do you see?http://wiki.mikrotik.com/wiki/First_time_startup

3

I do not have null modem cable just now, will post results once this is tried. Thanks for quick response!