In article , Ken Riley
writes

We are currently carrying in and out of the conservatory (out in the
evening into the warm living room, back into the light conservatory
for the day) a 12inch twig. It had leaves on when we bought it in the
spring and has had lots of flowers but by November we were singing
"four leaves on my lemon" then "three leaves on my lemon". Two weeks
ago the last leaf fell off and on Monday last the pathetic little
half-inch green lemon hit the carpet. Is there any hope for it? Are
we wasting our time carrying it in and out?

Probably, though it may re-shoot if you are very lucky.

Two things seem to go for citrus:

scale insects - you will see round shiny brown lumps abut 1/8 inch long
on the twig which can be pushed off with a fingernail

Red spider - any fine cobwebs on what remains?

Remove anything you can see, wipe the twig with a damp tissue, and hope.

--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/garden/