93 Cabrio will not turn over. Help!

Hello, I am an 18 y/o from Jersey who just recently purchased his first car, a mk1 93 Golf Cabrio. That being said, i know nothing about this car, but the moment I got it I started doing work.

It was running completely fine, maybe some strange things happening when idling, but otherwise, it ran smooth, and could easily take me where I needed to be.
Recently, I tried to replace the horn. I drove it to a garage my friend owns, and took out the battery. 
We jacked the car up, eventually took the horn out, and realized there was clearly some weird stuff going on from the previous owner. 
after messing around a bit (trying to plug the one connector inside the wheel to the wheel, but to no avail) we put the battery back inside, and tried to start it, but it would not turn over. The lights were on, the electrics seem to be working fine, but for some reason it simply will not turn over.
Any suggestions? I really want to get her back on the road.

The ground for the horn is on the right or left side steering rack mount to the Steering rack gear box.

The horns work as this:


While you were looking at the horns, on the right side frame horn, there is the main chassis ground point there.

That ties the battery negative post to the frame and from the frame to the engine or transmission.  If your cables are still oem, try my quick grounds test to eliminate a wonky ground.

https://www.volkswagenownersclub.com/threads/brianos-quick-grounds-test.134637/

Manual or automatic transmission.  But try the quick ground test, and that will tell you either your grounds are good or not and if they are the issue.

When doing the grounds test then try starting the car, if it works, then you need to replace your car.

Does your car have an air bag?

Is there an immobiliser fitted to the car?
When I unplug my battery and reconnect it I need to reset the immobiliser with in 1 minute of connecting the battery with the key fob or it will never start even if you switch off the immobiliser with the key fob.

I will try this solution– though two things: I do not have a key fob as it is a mark 1 cabriolet, and is essentially stock. Additionally, if this grounds test does not work, don't you think an entirely new car is drastic? I can't see this issue being so problematic that it would require an entirely new car. It was running fine before we took the horn out, and we put it back together the same way it was. We will do the grounds test as well as checking for any blown fuses.

My car does not have an airbag!

My car did not have a immobiliser fitted from factory, it's an aftermarket one so that's why I have a key fob.
Check and double check what you did as it was working before you messed about with things so you may of dislodged something.