I called my hosting company to make sure that they hadn't done any upgrades that would have caused problems. So it doesn't appear that it is from the hosting company.

I am not sure why it would have quit working when no changes have been made.

The only thing I have done on my site is add product since it last worked properly.
Same story here--everything was working. I couldn't make IPN work for echecks so I switched to paypal express. I used it would problems for a few days and then it suddenly stopped working. I've made no changes to the site-I've only added additional merchandise.