Paint.Net is a 3rd party ( non Microsoft ) program, see https://en.wikipedia.org/wiki/Paint.NET which gives further information and a website download link.
The website ends up here - http://www.getpaint.net/index.html
The latest version appears to be 4.0.10. Look carefully as to what link to click, its not always that clear, so make sure you are clicking for Paint.Net 4.0.10 ( or whatever the latest version is ).
As always*,* its probably a good idea to update your AntiVirus program and run a full scan of the downloaded ZIP file and the included install file(s) before actually installing them.
However, if you are still having problems after trying the new version, then as its a 3rd party program if its the only program on your PC that is having problems, it may in the first instance be best to ask 'Paint.Net' itself as to what the problem may be - http://forums.getpaint.net/