Any prescriptions about _not_ placing "then" after a comma?
I find it usually after a period or a semi-colon, or after a "comma and."
Thanks.
I would make it two sentences to avoid the semicolon, or insert "and" as 
you mentioned.
-- 
Stephen
Ballina, NSW
No prescription I have ever heard of.  Castings using it are really a 
matter of style, hence personal preference.  When the sentence is short 
and clear, it should be harmless.  Sentences with it often have it after 
a conjunction:
I watched the movie, and then went to bed.
It costs $20, and then there's sales tax besides.
By parallelism with those examples (from the AHD4), one could write:
"The thought hadn't been present, and then suddenly it broke in."
Or, perhaps better:
"The thought hadn't been present, but then suddenly it broke in."
As I said before, though, I think that in short clear uses, the 
conjunction can be dispensed with.  (Analogous to uses like this 
description of a musical presentation: "John sang, Mary played.")
What I find more ill-styled is the use of the parenthetical word 
"suddenly" without any setoffs.  I grant that--
"The thought hadn't been present, then, suddenly, it broke in."
--is not much better, but either of--
"The thought hadn't been present; then, suddenly, it broke in."
or
"The thought hadn't been present, then--suddenly--it broke in."
might be better. Or perhaps best of all, skip the issue:
"The thought hadn't been present, but suddenly it broke in."
-- 
Cordially,
Eric Walker
[snipping great stuff because of the server]
Great instruction:-)
Thanks, everybody.