No Crank No Start Security Indicator Light Flashing Rapidly and making clicking noise

My 2007 Mazda 6i had been running great and then after parking it
overnight, it wouldnt start the next morning.  It tried to crank at first then
slowly died out and then started making a clicking sound.  I did check to see
if my headlights worked and they were working at that time.  This issue
started about a week ago. Unfortunately I had to leave it sit for a few days
because I didn't have time to mess with it and now when I went out and tried
to start it I get basically nothing.  It won't crank at all so obviously it won't
start, and when I turned the key to see what would happen the security
indicator light started flashing rapidly and a rapid clicking noise can be
heard.  Also, now my headlights won't turn on at all and my doors have to be
manually locked and unlocked.  I'm pretty sure my battery is basically
completely dead now but what else could cause my no crank no start
security light flashing & clicking issue?? I sincerely appreciate any help with
this as I am very low on funds and I desperately need my car asap.  Thank
you!!