The adjective 偶然
The adjective 偶然 
is used to talk about unexpected situations. It is followed by the particle 的 .
is used to talk about unexpected situations. It is followed by the particle 的 .Examples
那次偶然的会面改变了我的一生
That chance meeting changed my life
nà cì ǒurán de huìmiàn gǎibiàn le wǒ de yìshēng
我认为我们相遇不是偶然的
I don’t think we met by chance
wǒ rènwéi wǒmen xiāngyù bú shì ǒurán de
这一偶然的事件使她立即想到了一个计划
This unexpected event caused her to immediately come up with a plan
zhè yì ǒurán de shìjiàn shǐ tā lìjí xiǎngdào le yí ge jìhuà
偶然 also can be an adverb "accidentally, by chance". In this case it is followed by a verb.
Examples
她在去附近一所学校参观时偶然发现了这个问题
She unintentionally came across this question while attending a nearby school
tā zài qù fùjìn yì suǒ xuéxiào cānguān shí ǒurán fāxiàn le zhè ge wèntí
她是最后一个才知道的人之一，偶然听到了
She was one of the last people to know about it. She had accidentally overheard it
tā shì zuìhòu yí ge cái zhīdao de rén zhī yī, ǒurán tīngdào le
我们偶然相会
Our relationship started by accident
wǒmen ǒurán xiānghuì