1998 Mercury Sable LS Car wont start!

Hi,
A donation would be a nice gift to you guys/ gals, however you are my only means for keeping my new job. I have no money and am living on a prayer. I heard about your web site from a autozone employee. Well that being said here is my problem. I replaced a dead battery in the car yesterday and now the car will not start. The car acts right until I turn the key and then everything goes dead no power and the theft light blinks repeately, until I disconnect the black lead wait for 5 min then it goes back to normal, lights work radio etc. Until I turn the key again. Can you please help me.