How can I fix my pc

So basically my brother was using my pc it has windows 10 the c drive got full and before I could do something he turned it off now it won't boot and goes to recovery mode where nothing works I had a restore point but it says my c drive is full so I can't use that and it won't allow me to even reset the pc with or without the files . Should I use a usb with the windows installed would that work even with the c drive being full??