Broken Molar - Causes + Repair?

Hi all,

At the end of last week, I noticed a sharp edge to my rear, left hand side
molar, 2nd from the back - not any particular pain. As my check up was due
anyway, I made an appointment for this coming Monday with my dentist just in
case.

On Friday night, after eating, a small piece of the tooth has broken away
from the edge of the molar, leaving my tongue to scrape at the sharp edges
left. It's fortunate my appointment is soon.

The molar has had a 'silver' filling in it, but I've not bit down on
anything obviously that's broken it, so can anyone advise why this may have
happened?

As the cracked piece is to the rear and is the side of the tooth, will a
filling work or will it need to be crowned? Can a filling be 'butted' up to
the edge of a tooth like this? There's no way I want to lose this tooth.

Any thoughts very welcome.

Thanks all,

Matthew