A new user with reputation less than 50 told me that she can see no "reopen" button under a particular closed question. Is it because of insufficient reputations that "reopen" button under closed questions cannot be seen?
Yes, you need at least 250 reputation to see and cast close/reopen votes on your own questions and at least 3000 reputation to do so on any question.
This is a screenshot I took of one of my questions on a site where I have less than 250 reputation:
New users can still cause their own questions to be sent to the reopen queue by making a good edit to their recently closed question and checking "submit for review". If that doesn't work, then asking on meta is the next step (even users under 5 rep can ask on meta as long as they are asking about something they posted on the main site).