The developers do their best to bring you a stable program. However, with many interacting features and several platforms to support, it’s easy to overlook the occasional issue. And, of course, something might be wrong with your system, too.
The first port of call is the Krita Artists forum: https://krita-artists.org/
This is a community of users and developers of Krita. Go there first an explain your problem. Mention your operating system (Windows, Linux, Macos, Android or ChromeOS), the version of Krita you have and your problem.
It’s possible that you’ll be told that you have found a real bug in Krita and that you will be asked to make a bug report. Here is a document that tells you in detail how to report a bug for Krita.
Do not mail the Krita Foundation or developers directly for support. Krita has millions of users, and only half a dozen to a dozen developers!