Why are my cookies coming out think and crispy?

So, I've used this recipe in the past, and it worked out really well, the best cookies I've ever made, but I made them tonight (for a church function no less) and they're coming out terribly! They're hard, crispy and thin. I tried not to beat the batter too much, and only cooked them for about 6-7 minutes (they're crispy on the outside, and not done at all on the inside) The only difference I can think of is that I'm making two batches at a time, and using a different kind of butter (I'm using margarine, cause it was the only kind I had at the time) Can someone please tell me what went wrong, and can I save my cookies??!!