Worth a try before you pay for someone else to fix it.
Pull the battery out and leave it out for 10 mins or so.
Put the battery back in, and hold down the Volume down button + Home key + Power key, in that order. If you still can't get it into Download mode than it's off to the repair shop by the looks of it.
If it does go into Download mode, try flashing it again using Odin.
All the best.
Ray