Blank screen DURING upgrade to El Capitan

Dear community!

I was doing the upgrade from Snow Leopard to El Capitan and the blank screen came. It's been more than 5 hours now, and counting. Should I wait longer? How long would be a very long upgrade? There is no progression bar and my Mac Pro is on but kind of silent.

I'm afraid that I won't be able to reboot, once the process was being completed. Should I then reinstall Snow Leopard using the discs?

Thank you!