Hi,
Try with connected phone to the PC to enter download mode. Check the Device manager is there connected device. If you can enter Download mode, then go here:
http://www.sammobile.com/firmwares/
to download firmware and tool for flashing (Odin) for your model.
But if you can't enter the phone in download mode and the PC can't see any connected device, you can try to enter the phone in download bode with USB JIG dongle like this:
http://www.ebay.co.uk/itm/Micro-USB-JIG-Download-Mode-Dongle-for-Samsung-Galaxy-S4-S3-S2-Note-i9100-i9000-/151458167134?pt=UK_Mobiles_Accessories_RL&hash=item23439c395e
Also you can find in the internet how to make by himself JIG dongle.
Finally, if you can't enter again download mode, the problem is HW and you will need a mobile phone specialist.
Regards