You can apply locks to a site collection to prevent users from updating content or you can temporarily prevent users from accessing the site collection.
To unlock/Lock a site collection follow the below steps.
- In Central Administration, click Application Management.
- On the Application Management page, in the Site Collections section, click Configure quotas and locks.
- Use the Select Site Collection page to select a site collection.
- Select Read-only (blocks additions, updates, and deletions) to prevent users from adding, updating, or deleting content.
- Type a reason for the lock in the Additional lock information box.
- Click OK.