网友不能脱离语境，孤立地去猜测哪一个是对的，哪一个是错的。实际上，这两个表达都正确，只是结构和意思各不相同而已。
as it happend：——方式状语。it 指代上下文提及的事物。（还有可能是：时间状语从句或原因状语从句）
as happend：—— 传统语法说的 “非限制性定语从句”，as 指代整个主句内容；现代语法说的“比较从句”。

看例句，对比如下：
一、方式状语从句：
She told me the whole story exactly as it happened. 
她把发生的事源源本本地告诉了我。
《汉英大词典》
He called Amy to see if she knew where his son was. As it happened, Amy did know. 
他打电话给埃米，看她是否知道他儿子在哪儿。碰巧的是，埃米还真知道。
《柯林斯英汉双解大词典》
As it happened, the bite was virtually painless: more the kind of small bite you might get from a naughty child. 
事实上, 这种咬几乎是无痛的：它类似于你被顽皮小孩咬一口的那种程度。
高考真题- 2015 湖北 阅读A
As it happened, my advice proved to be right. 
可巧的是（正如事情发展的那样），我的忠告证实是对的。

二、一个特例，是原因状语从句：

As it happened so many years ago, I don't remember the details. 
因为年岁久远，当时的具体情况已记不清了。
《汉英大词典》

三、还有一个特例，是时间状语从句：

As it happened, people began to realize its importance.
随着事件的发生，人们开始认识到其重要性。

四、非限制性定语从句或比较从句：
As Happened in Jamestown, about half the settlers in Plymouth died the first winter. 
与詹姆斯敦的情况相似，在第一个冬天，普利茅斯殖民地一半的人去世。
As happened with so many theater actors, he was swept up in the vortex of Hollywood. 
象发生在许多戏剧演员身上的一样，他被卷入了好莱坞的旋涡。

网友如果仍然有疑问，请写出具体的句子，再给你详细分析。

as it happened  碰巧；恰恰；令人奇怪的是：

As it happened, the bite was virtually painless. 令人奇怪的是，这种咬伤实际上不痛。2015 湖北卷阅 A

(摘自 陈根花主编《高考英语备考1号  词汇编》p.219)

as it happened 是一个副词性短语，通常用于谓语动词是过去时态的句子中。如果谓语动词用现在时或将来时，要用 as it happens。如：

as it happens  <adv> 恰巧地, 凑巧地, As it happened, he was at home when I dropped in. 偶然地 我顺道去造访时凑巧他在家. As it happens, I have no money with [or on] me now. 我现在身上刚好没带钱。(摘自英汉多功能词典新编升级版P.757)

As it happens, we probably won't have all that much time for a holiday, though. 但是，碰巧我们的假期不会有那么长的时间。(摘自赵振才《英语常见问题解答大词典》增订版第五版P.158)

as it happens  也可以用于过去时的句子中。如：

I saw him yesterday, as it happens (= It (so) happened that I saw him yesterday.) (摘自张道真主编《现代英语用法词典》修订版P.681)