Can't mount /system, Odin doesn't recognize phone

Hope you guys can help me, I ran outta ideas. Here's my problem: I was on a cm12s based rom, downloaded latest SlimRom and when I was installing (using twrp recovery) it gave me an error: "Unable to mount /system". I read some stuff on different websites and ended up trying with another rom and other Recovery's (philz and cwm) but none of that helped. After that I downloaded stock rom to install via Odin but Odin doesn't recognize my phone, I installed Samsung USB drivers, installed kies, tried with different usb cables, tried with different computers but Odin simply doesn't recognize it, now my phone is stuck at download mode since I don't have os installed, is there any way to fix unable to mount system problem? Or to make Odin recognize my phone? I would really appreciate your help.