This description of the issue may apply to a few different scenarios. Please review the following options for the best solution.
1) The student has attempted to log in with the wrong password too many times.
Teachers/Admins: Access Issues: Account Locked
Students: Student Access Issues: Troubleshooting Overview
2) A teacher has blocked the student's access using the Controlled Access feature.
Teachers/Admins: Controlled Student Access
Students: Why is my account blocked by my teacher? (Controlled Access)
3) Stage 2 remains locked, even though the student has reached 90 on Stage 1.
Teachers/Admins: How to Manually Unlock Stage 2
Students: Please talk to your teacher for assistance.