My HRM strap went once, and I tried to change the battery to no avail, I still would not pick up a signal.. I took advice for the net/ RW forum, and it said to put the battery upside down from its normal position, for a certain period of time (cannot remember how long, but it wasn't ages). Then put it in the correct way around.
I tried it, and it worked. It was something witchcraft-y to do with polarisation and negative fields or something like that. There was an explanation about it, but I couldn't understand it.
It worked that's the main thing... I have always found good advice re HRMs and watches on the RW forum/ other runners forums.