iOS 7.1 update from a PC causes my phone dead
Hopefully you did a backup before updating? Right? ;-)

If you have a backup file you simply plugin the iPhone to your computer and follow the last step in this guide ("Restore from a backup").

If you didn't make a backup, try to put your iPhone in DFU mode (guide here) and follow the iTunes instructions to restore the iPhone to default. (You will lose your stuff if you haven't backed it up earlier)