。
She was so tired that she couldn't walk.
Literal
She [topic-は] very tired-was [because-ので] could-not-walk.
疲れている is the standard way to say 'is tired' — Japanese treats tiredness as a resulting state, not a snapshot adjective, which is why the ~ている form is the default. The past 疲れていた preserves that state-frame in the past. 歩けない is the negative potential of 歩く ('walk') — 'cannot walk.'