We came across this issue last month after patching our SharePoint servers, after all the servers came back up after reboot , users tried to upload a document onto a specific site within our SharePoint and they didn’t have any option to at all, no matter what account we were logged in as it was not possible.
It was actually not possible to make any changes to the specific site at all, however we had 2 different SharePoint sites on this SharePoint server and they were fine.
The issue in the end was that the website had got locked down into read-only mode and we just had to simply change this to not locked.
1, Open up Central Admin
2, Go to Application Management
4, Open up ‘Site Collection Quotas and Locks’
5, Change the site collection to the site your having trouble with and simply change the site to not locked.
6, Now change the lock status to not locked and ok this change and your site should be fully functioning again :)