English has/had so many of these trios: where/there/here, whereby/thereby/ hereby, whither/thither/hither, whence/thence/hence ... and when/then/NOW? Whatever happened to "hen?" Did that form exist in an earlier stage or dialect?
Yes, it did.
From Middle English henne, heonne, hinne, from earlier henene, heonenen, henen, from Old English heonan, hionan, heonane, heonone (“hence, from here, away, from how”), from Proto-Germanic *hina, *hinanō (“from here”), from Proto-Indo-European *ḱe-, *ḱey- (“this, here”). Cognate with Dutch heen (“away”), German hin (“hence, from here”), Danish hen (“away, further, on”). See also hence.
EDITED TO ADD: To be sure, this answer addresses the "Whatever happened to "hen?" Did that form exist in an earlier stage or dialect?" part of your question. hen did not really mean 'now'.
The answer to your question is no, and, what's more, the deictic (IE *ke > hen)-related forms that Mark Beadles refers to cannot be directly connected with the bound morph -en of Modern English then because -en (just as -ere, -us - as in thus, etc.) is not deictic in then, there, thus, etc. What is deictic in all such complex words is the bound morph th-, not the -en (-ere, -us, etc.) that follow it. And, of course, the reason why now does not fit in the pseudoparadigm attempted above and why -en can never have meant now in English, is that now is deictic, whereas -en cannot be.
This impossibility can be explained in a principled way: a word (or phrase) containing two deictic elements, be they free or bound forms, would violate a universal principle, i.e., that variables (say a time variable, as in th-en) cannot be doubly bound. Hence, no deictic can possibly 'bind' (read 'apply to', if you prefer) a constituent that already contains an internal deictic: once the (time/place, etc.) variable is bound, a second deictic would have no variable to bind, biuniqueness would be violated, and the resulting word/phrase would be necessarily ill-formed.
That is, in short, why then can not be analysed as [t(h) + -(h)en = now] and why the answer to your question is No, English could never have used -hen, (-en, rather), to mean now, even if deictic hen-related forms did exist in, presumably, all primitive IE languages, as Mark Beadles pointed out.