The headlights, horn and power door locks are not working

It sounds like you had a voltage surge when you reconnected the battery cables which happens, let's start by checking all the uses in the under-hood and interior fuse panel. here is a guide to help with the fuse locations in the diagrams below:

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.