layer becomes invisible

I am not able to have the check mark in the layers box to make it visible.
For some reasons all I can get is a dash for the group (even though I can "check mark" inside the group.
I don't get this blue color that tells me that the group is active. All I get is the light grey...

How do I turn on the layer if all I can get is a dash, and not the check mark?

A dash on a group means not all the layers in the group are active. Sounds, though, like you have accidentally soloed one or more layers or groups. Select all your layers and groups, go to the Object menu, and choose Unsolo (can't remember what it's under in there, but it's there)

not really the solution...
what i've been doing is to reimport whatever I had in the layer...
For exemple a Pic's icone which is usually bright green, would become shady green... And I when I reimport, it'd be the regular green