serious matter; grave affair; big trouble
「大事」 has another reading with different meanings.
important; serious; crucial
important; essential; significant; major; key; principal
大事にならなくてよかった。
I'm glad it didn't become serious.
小さなミスが大事になった。
A small mistake became a big deal.
これは大事だから気をつけて。
This is serious, so be careful.