@irfon You don't give us any description on 'the situation' when this occurs. Just after you have eaten a strawberry pie, or had a coffee? Just joking.
The icons are all rendered through a font file, I think fontawesome and there is another font file as well filled with plone specific icons.. If you want to debug this, check if/why the font resource is not loading in your site.
I've seen this issue before when you have a customised theme where the reference to the font file is loaded using a broken path.
This might happen if you load a resource from css on /++theme++resource/etc It works when the site is hosted in production on http://mysite.org/, but when you view the icon file locally on a development machinie on http://localhost:1234/my_plone_portal/ , /++resource++ will be called on the zope root and not the plone portal route.