The triac probably went short-circuit if the old bulb blew.
It is usually a metal-tabbed device such as a TIC226.- Hide quoted text -

I had one fail in exactly this way. The triac shorted as a result of
the very high current which flowed when the bulb blew and arced
internally with a bright flash and a pop. I looked up the data for the
triac. It did not have a surge rating anywhere near high enough to
survive this kind of fault, especially with the lamp close to the
consumer unit (the lamp was in the hall in our case).

B&Q replaced the first one, but refused to replace the second which
was just over a year old when the bulb blew. The manager insisted
that nobody else had ever reported such a fault and anyway they were
not obliged to replace goods more than 1 year old.

I made a token fuss in the shop, but it wasn't worth pursuing it
further for £12. I used a switch after that.

John