If you are getting the message 'Assessment locked' you are likely seeing a page like this:
This means you must complete all the pages that aren't complete.
In this case, the complete pages are the fully filled in purple stars, other styles of courses could show the completed pages as a tick in a circle.
Once all the pages are complete, the assessment will unlock itself and you will be able to go back through it.
Don't worry there is no limit to how many times you can go through the assessment, as long as your course hasn't expired, you will be able to go through the assessment.
Completing an assessment more than once will not alter the original completion date.