考えられない【かんがえられない】

考えられない is the negative potential of かんがえる考える: "can't be thought of", so "unthinkable" or "I can't believe it". A whole clause in the plain form goes in front with なんて or とは; a noun takes は or が, as the object of any potential verb does.

なんて carries surprise and sounds conversational. とは is cooler and fits writing and careful speech. A clause that ends in a noun keeps だ before とは: あのひとあの人がはんにん犯人だとは.

No other possibility

With ほか他の or いがい以外, 考えられない says there is no other explanation or option you can think of.

Out of the question

Applied to a choice you might make, it rules the choice out.

The affirmative, かんがえられる考えられる, is "it is possible / conceivable" and has its own grammar point. 考えられない changes like any ない form: かんがえられなかった考えられなかった in the past, and politely かんがえられません考えられません, かんがえられませんでした考えられませんでした.