Do you have access to another Windows system? If you do you can salvage your files using a SATA to USB adapter like this one: [https://www.startech.com/HDD/Adapters/USB-3-SATA-adapter-cable-with-UASP~USB3S2SAT3CB|Startech 2.5" SATA to USB adapter].
Follow this YouTube vid on removing your drive: [https://www.youtube.com/watch?v=QlkKTYAPvy0|Toshiba Satellite L855 / L855D Series: HDD Replacement].
Connect your drive to the adapter and then onto your working PC. transfer all your files over so you can then wipe the drive and try to re-install the OS.
Hopefully you still have the CD that came with your system. If not you'll need to order it from Toshiba support as it has the needed drivers for your system.
You may want to consider replacing the drive as your old drive maybe on its last legs.