We have an issue where site collections get set randomly to read-only.
To get it unlocked is simple:
Go to Central admin, Application Management, Configure Quotas and Locks
Find your Site Collection and set it to Not Locked
Ok, not how do you get a solution for this...
Firstly have you reached your Site limit or Points limit as in settings above?
In my case no...
The other issues that can cause this is Interrupted Backups and Broken Web Parts
Lets Start with Interrupted Backups.
There is a few ideas going threw my head on this one...
Firstly we have hourly log backups, does that maybe interfere with the daily backups sometimes.
Then out Backup Admins also backs up with Backup Exec. Does that interfere.
Then there is site collection backups via SP PowerShell Scripts, all successful.
Did some more research and found one guy that said they got this issue on Big db's even after backups was successful.
Their solution: Run this script after each backup.