2001 Toyota Tacoma - Battery swap gone wrong

My friend has a 2001 Toyota Tacoma, we took his battery out to see if it would work in my 2006 Cadillac.Its an Odyssey Deep cell battery) we put his battery back in and now it wont turn over and his headlights are on without the head lights being turned on. What could the problem be?

Installing the battery with the polarity reversed will cause problems and damage, check all of the fuses.