There are four possibilities:
1. 7805 has been damaged;
2. The connection to 7805 is incorrect. Connection method (TO-220 package) : three welding feet down, look at the front of the word, from left TO right are (1), (2), (3) feet, (1) feet for input; (2) The feet are the ground; (3) foot output;
3, the nominal voltage of 12V battery output voltage is lower than 5.5V, when the input voltage is lower than 5.5V, 7805 will not be able to constant voltage saturation output voltage reduction 0.7V less than 5V output, internal adjustment tube 0.7V, so the input must be at least 6V above;
4, 5V end output load exceeds the original 7805 power rating. The absence of one of the above four conditions will inevitably get no normal output voltage.