Blender not associated with .blend files.

This keeps going on and off, one day blender will open the .blend when I double click on them, the next they are all unknown files and I have to manually go into blender and open it. Anyone know what the deal is?

This is likely to be more a system problem then a blender program problem, i think you’ll need to add some info on what operating system your using for people to be able to help out.
There could be another program you use that also uses .blend extension