Engine won't even turn over! What do you think?

Battery vdc=12.51, it is a new battery & works fine. Dash lights come on. @ Other points of interest1. Car was sitting for a # of years. I just recently got it back on the road & insured. Trans. Was completely rebuilt & works great. The speed sensor started a few weeks ago & the no crank happened >I removed battery cables & touched them, as was mentioned. 2. There has been times when i'd go to start eng. & Nothing would happen. I'd turn key Off, then try to start again & it'd crank right up! It always starts quickly--never has to turn over a lot to start! Engine runs strong, all lubs good, and is very reliableexcept now!
Randy

Well, I replaced the s/w a few yrs. Ago because of mech. Issue w/it. I keep going back to the fact the engine did start Always, but now > touching those battery cables together (off the battery), it won't crank at all! There Must be something (electrical?) That didn't reboot or charge up or? > Reconnecting the battery! The neutral s/w worked just fine also > trans. Rebuild, ----plus car Should start in Park, shouldn't it, even if neutral s/w Was bad. It always gets started in Park!
Randy

Okay, as soon as I read about the alarm system, that I thought is the culprit and it was! Now I want you guys to know I really appreciate your help in fixing this issue. But, I may still have the Speed Sensor problem. I won't know until I drive car a bit. Now, looking back from my first email where I mentioned what was listed to do; remove and replace battery cables, touching leads, the information about "alarm system". Should have been included in the statement! I was pretty sure (or at least hoping), it was something simple, since everything on car worked just fine, with exception of speed sensor and none crank at initial try Sometimes. So, with all that said, I again thank you all for your help and will get back in touch. Driving car around to see if speed sensor/speedometer/tachometer, is working full time or intermittently. Randy in Fla.