SharePoint Site Collection Lock and Unlocking

Reasons for SharePoint Site Lock (Read only Mode)

1>Site Collection Backup cancelled or interrupted

2>Broken web part locks out SharePoint site

When upgrading a custom web part in SharePoint, it is REALLY important to uninstall the previous version of the web part and its referencing web pages or risk locking out all users including the Administrator from your website.

Impacts on the SharePoint site:
1> Site Collection will be turn into Read only mode.
Nothing can be saved.
2>All Farm administrators and site collection Administrator will get access denied on several pages like Create.aspx and AreaNavigation.aspx
3>You cannot able to add any user and not even you wont see the option to add user.
4>If you try to open through SharePoint designer it will open in Contributor mode(Content Author) and you cannot able to See code view and cannot copy or Modify any thing

Solution:
Go to Central Administration > Application Management > Site Collection Quotas and Locks
Select the Site Collection and chose the correct option to unlock the site as below.
Lock status for this site:
Not locked -- Select this option to unlock
Adding content prevented
Read-only (blocks additions, updates, and deletions)
No access

After Selecting the Option Not Lock, the issue will be resolved and you can able to view all the options :)