k550i master reset

Hi so I have a problem with my phone, I was using w610 firmware and everythink worked great, but than i got my fastport problem k550i fastport - Top Sony Forums ad so i want my old FW back to have it repaired under warranty. I have backup so i uploaded it back, and i wanted the phe clean so i did master reset, and thats not a good idea. I ended up with select and ok in the middle of the screen and nothing else.. (so i did reupload my fs and its working again) i want to know why did this happen. Thx