i have played about 20+ hours and I found there's some door and chest are locked and need someone called Sandra to unlock it . To find her I need to go to dungeon under the snowfort .... But I was searching about 2 h but didn't find her. The only place I couldn't pass are the bridge with guard , a giant ball stone block after I saved the guard and the trap with 3 bowling stone . Did I miss something or I did something wrong ??

I assume you already met Sandra in the dungeon earlier but picked the option to leave her to the soldiers. In that case she is moved to Ironholm Prison in Begus, where you can still recruit her.