2001 Chevy Tahoe LS Starting problems

Need some help. I checked the battery. When I try to start my truck nothing happens It will not even crank nor does the starter make any noise, but I am still getting power to the rest of my truck. What could be the possible problems why my truck is not starting. The funny thing is this past weekend I got in to start it on saturday and it did just as i described above. I checked the starter cables, battery connections, still nothing. I was leaving for work this morning, Monday, and thought I would see if it would start and it cranked right up, just like nothing was wrong. During the weekend i probly tried to start it 20 times and nothing happened. PLEASE HELP.

first of all you need to see where you are losing power, easiest to back track, start at the starter wire (purple) when the concern is present of course or you will be wasting time. next would be the starter relay, try swapping the relay with another one in the underhood fuse block. also make sure the positive battery cable is not leaking acid. i would remove it to be sure.

what is the security light doing when this happens?