i changed the battery in my truck now it wont start

Did you check you ground connection? Also the connection to the starter? Some times these play are roll. What kind of truck, year too?

This sounds like a security issue. Do you have an alarm or a security come with the truck?

OK, this is going to sound silly, but it actually happened to a friend of mine. He left the plastic cover on the post when he replaced the battery in his boat. The clamp crimped it just enough that the lights would come on, but it wouldn't turn over. So, I would say take the cables back off, clean them and make sure they are tight when you put them back on. Other than that, put a battery charger on it to make sure the battery has an initial charge. If that doesn't work, let us know some more details. (Did it work before you replaced the battery? Or did you replace the battery because it didn't turn over?)