Pointers on where to look for core problem

I have until (about) today had things working okay.

But I have just stop receiving email notifications from cs-cart and also payments that have been made (through paypal) are returning marked as a failed payment. But they have actually gone through.

Seeing as how both these seen to have happened at around the same time I was thinking that they might be related and was hoping that some of you that go under the bonnet more might be able to suggest what might have gone wrong and where to start looking.

All advice welcome and thank you in advance.