Car wont crank

I just unclogged the ac drain tube that was causing
water to accumulate on my floor boards, before
doing this I completely disconnected the battery.
After I was done with the draining I went to
connect my battery and tried to start the engine
but no start... just a clicking sound and the dash
pulls up a "check battery charging system " and a
"open and close driver window" any ideas of what
could be going on???