Had you recently treated the snake for anything? Mites or anything? Did it come in contact with any chemical? What kind of substrate are you using?

If the warm side of the enclosure is up around 90 degrees and the ambient is 80 or higher then there is some other reason the mouse was not digested.

I asked the other questions because sometimes after treating for mites or infection etc.. They can get the "runs" and the poop might look like a half digested mouse. If it is clearly an undigested mouse then either the snake is sick and could not properly digest it, or it was not warm enough to digest. Maybe there was something wrong with the mouse?? IDK

Whatever you do, if it was a regurge, do not feed the snake for a couple weeks. Give it a chance to recover before attempting to feed again.

I hope you get it figured out.