How to Solve:
WE APOLOGIZE FOR ANY INCONVENIENCE, BUT WE’VE MADE THE SITE READ ONLY WHILE WE’RE MAKING SOME IMPROVEMENTS.
I came across an interesting issue recently where our production sites flag notification for every user "We apologize for any inconvenience".
Symptoms:
This issues for regarding scheduled a backup task scheduler failed or interrupted for any resign,
I my case at the time of scheduler drive not have sufficient space. backup process was abnormally terminated.
This resulted all Sites being in Read only mode & the following message would show up when users browsed to the site.
We apologize for any inconvenience, but we’ve made the site read only
while we’re making some improvements.

Go to “Site collection Quota & locks” in Central Admin & here is how the Status looks like. It is locked as “Read only” & all settings are grayed out weather you have administrator permission.

Here is more on this behavior & how to get out of this situation.
In SharePoint 2013, Microsoft introduced a property MaintenanceMode
for SPSite
object which indicates the site is undergoing a Maintenance & is read only. SPSite.MaintenanceMode
flag can be set on a site for several reasons like content database is in read only state, or site collection is being upgraded, backed up or moved.
If a site gets into a state where the action that set this has terminated in a way where this is still set, we run into this situation.
Solutions:
The way to clear this flag notification is use the ClearMaintenanceMode
method in SpSiteAdministration
object. Here is how it can be done via SharePoint Management Shell.
Option Command 1:
#Run this command search site separately or run one by one all top level sites if we have multiple top level sites/ Web application.
# SiteName “http://m.com”.
$Admin = new-object Microsoft.SharePoint.Administration.SPSiteAdministration(‘SiteName’)
$Admin.ClearMaintenanceMode()
$site.MaintenanceMode
Option Command 2:
Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue
#Clears all the maintenance modes for all Site Collections in the farm.
$webapp = Get-SPSite -Limit ALL
foreach ($site in $webApp){
$Admin = new-object Microsoft.SharePoint.Administration.SPSiteAdministration($site.url)
$Admin.ClearMaintenanceMode()
}