Why won't my car start I did a transmission change n hooked everything back up and it has never had a problem starting

If you installed a new manual transmission, or simply replaced the clutch, you may need to adjust the clutch pedal height--it may be preventing the neutral safety switch from working. Also, check all under hood and under dash fuses, using a test light or multi-meter. Finally, ensure the battery is fully charged and that all grounding straps are connected.

... and if it's automatic, doublecheck the Neutral Start Switch's connection.