Q: Magic trackpad has died

From your posting it was not clear to me if you replaced both batteries or just one of the the 2 batteries. Just asking. Maybe double check that you have two new batteries inserted so the polarity is correct. Screw the cap back on then hold the power button down briefly. Does the green light come on? Maybe then try to go through the pairing process again. Maybe try the steps at the link below.

If your Apple wireless mouse, keyboard, or trackpad aren't working as expected - Apple Support

You're welcome. One thing I forgot to mention... Some people have found that some brands of batteries are slightly different in length than others and, the Magic Mouse in particular, can be fussy and not work if the batteries are towards the low end of the length range. I've not heard that be a problem with the track pad before, but maybe your batteries are a little shorter compared to others(?).