Cat has not pooed in two days tomorrow will be three

[taram]

When do I call the vet?
I've been panicking about my cats a lot recently, not sure if I'm worrying too much

[Coffee Mugger]

How old is the cat?
Do they have any other problems (diabetes, obesity, kidney problems or even a low grade murmur)?
Have they thrown up in the last few days?
What food are they given - and is it wet or dry?