I have had this issue popup a couple times in the last 2 years and it always catches me off guard!

You notice that "all of a sudden" one day that your SharePoint Site is Locked. You can't add anything, change anything, like users or documents. It's all locked! This is usually because a Backup was stopped (or died from a server reboot!) before it was able to complete!

Simple solution that you can run at the SharePoint PowerShell Command Line: Set-SPSite -Identity "<SiteCollection>" -LockState Unlock
More Info here: http://technet.microsoft.com/en-us/library/cc263238.aspx