I had to change my battery after my blazer sat for a week when i changed it, it will crank but not start
If it ran just fine before you worked on it, the most likely cause would be something did not get hooked back correctly. Double check your work and insure there is no corrosion on the connections.
You may need to cycle the ignition key on and off to prime the fuel system. Setting for a while may have allowed the fuel pressure to bleed off.
Try spraying some starting fluid in the air filter, if the engine tries to start, you know it is a lack of fuel causing the trouble. If not, it may be electrical.