Momma once asked me the question "What would life be like without pooping?" and she mentioned poop magically disappearing and I told her "Very painful"
Here is what I think life would be like if for some reason the anus was closed and could not open again:
First day: Normal
A few days later: Extreme rectal discomfort and impaction
A few weeks later: Pain all over the abdomen(The whole colon is affected)
A few months later: Severe appendicitis(The appendix is totally blocked by this whole colon of poop)
A few years later: Malnourished and about to die(The whole small intestine is affected)
If somehow the creature could not die at all or had all kinds of IVs in it to prevent death, the stomach would get overfull and if not already from the appendicitis, the creature would vomit all the time. Stomach acid would quickly be depleted causing achlorhydria. Then there would be chest pain and a sore throat from a full esophagus. And finally it would get to the point where the creature can't even bite because the mouth is so full.