Best way to express that something is lost.

Dear community,
i am new to learning Japanese. I am unsure if i am too novice for this forum, but ill try.

So, i basically want to express that i cannot find something at a given moment, e.g. my keys. I want to start with ....
無くした と 思います. I think i have lost (it)....
and want to continue with 見つける

Is it now
i) (もう ?) 見つけません (~ I cannot find it)
ii) 見つけられません (~ I lack the potential / I am unable to find it (because its maybe stolen)
iii) 見つけて いません (~ I currently cannot find it, but i am still looking).

Or is it all wrong?

Thank you very much, and my apologies, if this question doesnt meet the forum standards.

X は見つからない

(The intransitive counterpart to 見つける is 見つかる).

Also, the following expressions work fine.

鍵が見つからないんです。
鍵をなくしてしまったんですが。
(が at the end of the sentence gives a nuance that you want to help.)

Thank you. Ill try my best