Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means frozen, so the vocab version is to freeze.

The reading actually comes from the word 氷, which means "ice." You can kind of see where the origins of these words come from, now... really cold frozen things. So, think back to こおり, then change the り to a る and put it outside the kanji. That makes the reading for this こおる, in all (or most) of 氷's glory.