Phone asking "SIM network unlock PIN" after flashing new Stock ROM on Xperia S I have Xperia S (LT26i). 
Earlier it had 6.2.B.0.211 installed.

I flashed it with "XperiaS_LT26i_6.2.B.1.96_1257-6919.ftf", flashing was successful, but after I started my phone I saw this message "SIM network unlock PIN". What should I do now?  

(Click the image to enlarge it)
Try this...

Link to LT26i firmware

You might have flashed different firmware that's the issue or Your phone might be factory locked... Try the solution in the link provided... :-)