Hello.
A few days ago i updated from blender 4.1.1, to version 5.1.2, and i’m having issues with hair particles.
For one of my models, the hair particles are not showing in viewport, and for another model they do show, but are coloured gray instead of the colour of the material.
For both models the hair gets rendered as it should, the issue is only in object viewport.
There has been two years of development between both releases.
First think to check would be to know, if hair is invisible only for old files, or if you tried to create them from scratch, too.
There has been Vulkan development in all releases. That could be a display issue relative to it.
In this case, OpenGL as Backend could be satisfying.
Most of announced compatibility breaks happening between end of previous series and start of current one.
I would try to open file in 4.5 and save a copy to try to open in 5.1.
If it does not work, try 4.2, the other LTS.
If it does not work, try to identify the specific release where it stopped to work.