What you may be experiencing:

• the sites permissions don’t appear to be working / broken
• site administrators / contributors can’t make changes
• unable to restore a copy of the site
• no errors are being reporting in any of the servers log files

Cause:

A failed backup usually causes this issue to occur, before a backup of your site is taken it  is temporarily placed into a read-only state / locked state, if the backup fails the site is not unlocked for normal day-to-day use

Resolution:

There are two ways in which to remove the read-only sitelock

Using Central Adminstration

Goto ‘Central Administration’ on your WSS3.0 server
Select ‘Application Management’
Under ‘SharePoint Site Management > Site collection quotas and locks’
Using the drop down “Site Collection” move through each site and check the “Lock status for this site”
Any site showing locked change it “Not locked” and click ok

Using stsadm

stsadm -o setsitelock -url http://server_name -lock none