Wisdom tooth, very old filling fallen out or part crumbled tooth.

[BCSurvivor]

Three months ago I saw the dentist for a regular check up.
I was in the chair for a maximum of two minutes and she seemed to hardly touch my teeth.
Everything was fine.
About 6 weeks ago I thought my very back wisdom tooth - with a large, 20 year old filling in - felt ever so slightly jagged at the front, but there was no pain and it was very difficult to see properly, due to position.
The filling was spread over most of my tooth, as I remember the dentist saying at the time that as my jaw isn't very wide it would be very awkward to fill.
A fortnight ago the tooth seemed to have more of a dip in the middle, but as it was still covered in silver and wasn't sore I ignored it.
But this morning, although there is still no pain and my gum seems fine, the back part of the tooth seems to be half missing, and there'd a definite hole in the middle, still with some silver.
I'm not sure if the tooth has crumbled at the back, loosening the filling, or whether the filling has just fallen out, as I can't see it properly.
I'm going to ring the dentist tomorrow, but am wondering if Im likely to lose the tooth or have it refillled, and whether, due to my narrow jaw, it may need to be done in hospital?
Has anyone had similar?

Mine was similar. Basically the back part crumbled away and then the filling fell out. Forty year old filling! Admittedly I did wait several months between the crumbling and seeing a dentist. (While other story). The tooth was basically dead and I had it taken out. Had lost three of the four ‘corners’ by that time. It was fine, but ultimately not saveable. Don’t really notice it’s gone.

small mouth but all done in the dentist chair in under ten minutes. No pain.

@Alphabet1spaghetti2 Thank you so much for the reassurance, I think I'll have to bit the bullet and ring the dentist!