Why don't this car start?

Hey Guys, Been trying to help a friend fix his 2000 Honda Accord but I've ran out of idea's. I've tried hooking my car up to his with jumpers to jump start his but nothing. Originally the car was working after a jump start but the next day it won't start again , the same day I figured that out I noticed the horn was sounding continuously whenever I hooked my battery up to his with jumper cables (also noticed it would spark when I put the negative terminal on. After testing a putting a new battery in his car it still wouldn't start. So I remade the ground wire, installed it the way I took it off, and placed the old battery back in the engine bay because it was sitting in the trunk for a few days till I had the time. Now that car won't do anything at all. When you put the key in the reminder sound when you have the driver door open doesn't ring anymore, and no lights come up on the dash when you turn the car into the on position before you tried to start it. So my question is the battery completely dead from sitting in the trunk? Its freezing in Ontario where I live. Is there a fuse gone that won't allow the car to start do to the ground wire being broken? Is the horn causing a parasitic draw on the battery? I've taken high skill major Automotive in high school, have an apprentice license looking to start college so I have some knowledge but I'm stumped. Please give me an idea.