A locked door can symbolize opportunities that you have not worked towards yet, or opportunities that you haven't been able to achieve. Or, a goal you haven't been able to achieve.
Explanation:
A locked door. Locked doors are doors that need a key to be unlocked, right? So, if the opportunities/goals that you have are behind that door, you need the "key", which is the hard work and effort you put into achieving that goal.
