What causes that alter becomes no longer accessible?

At lest one person from this forum reported that some alter went away, sometimes resulting in fronting as unwilling alter; something like that happened to me for 2nd time now.

For a week so far, I am no longer capable of accessing qualities and characteristics of alter I usually co-front with, hence I front alone and I am struggling more because of that.

What causes such so I can learn to prevent such?
How can one reawaken the alter that went away?

Maybe something happened when they were last out? …do you have other alters you can ask?

It seems there is no general answer to this question. In my case, it depends on my emotional state that all alters share. When there is emotional disturbance, I end up getting stuck as alter named Toni, and other alters are barely or not accessible. This time, I merely did something I am not proud of, where failing to move one makes me stuck as Toni, repressing the alter I co-front with.

Unfortunately there is no way to prevent getting front stuck or void stuck (our personal term for an inability to front) the pattern we have found is we get front stuck when life has ment we have needed a perticular alter for several days in a row which results in connections becoming weaker for everyone else until we loose the ability to voluntarily switch front (front because we want to) the most we can do is observe pasivly.
When we get void stuck it is generally because something has triggered a repressed memory for that alter and they are literally hiding, the trauma is sitting between them and the front and they can't get past it.

So the question is are you front stuck (to strongly attached to the front) or are they void stuck (incapable of perceiving the front let alone accessing it)

When one of us is front stuck we have to make an active decision to not, it's like being back a square one of discovering the system. You have to really listen for those random food cravings or act on that impulse to vaugly thumb through a book you would never read yourself because each time you eat that gross pickle or feel like you need eye bleach your connection gets weeker and their connection gets stronger untill there is a loosening of your hold on the front. And that first switch may only be for a few seconds while they enjoy the 5th pickle (thank god you no longer have to taste it) at that point we generally have a 5 minuet nap. Putting the body into a unconscious state while the other alter is vuagly nearby with a tenuous grip of front gives them the opportunity to take the reigns. It doesn't always work straight away, for us I think it can take 3 or 4 cycles of indulge the alter then nap before a switch is made and it won't always be the alter you want to switch to that is strong enough to break through but you have to let go because once they are front and you are back inside it will be easier for the others to replace them.

As for void stuck until you can eradicate the trigger and do grounding work with the alter you arnt going to reach them. The trigger may even have gone but if the alter doesn't feel capable of processing the emotion externally they may not front untill they can and the longer they are trapped behind the trauma the less likely they are to feel safe to front. You have to actively make a safe space externally, set aside time, lots of time and comfort items and then go sit next to the trauma. Let the emotions bleed into you and cry/rage /shake until the hormonal release that comes from performing an emotion reaches the trapped alter. Then things will (in our experience) get intense as they switch and you are dragged into the full intensity of the fear and you will instinctively want to ground and calm and sooth but that could push them right back into the void, especially if as a kid you had to hide your emotions. Let the freek out happen for as long as you can bear it then just before you get overwhelmed and pushed away start to soothe tell them they are safe express your appreciation for saving you from all that, give them love, reach for the comfort item, do the grounding.

Make sure you know each others positive triggers they really help when you are trying to "summon" an awol alter.
Learn which alters are each others cryptonite. is their another alter that can act as a chaperone or buffer if one needs to front after the other? (our mute alter weekens in the presence of our teen alter, their personality just clash, so generally we get one of our littles to butt in in their cute but cheeky manner for a few seconds before the mute alter is needed)