Website file replaced with other file(with a single line message) at random

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C#

ASP.NET

I am a freelancer ,I had created a web project with my partner ,later my partner left due to some issues ,after that website shows a message on home page that(actually homepage replaced with following message)

'The trial period of this website has been expired. Please contact your system
admin.'
but other pages are still working fine .When I noticed ,I came to know that when I rename the home page, problem goes off but again after somedays it happens again(this no replacement just got the above message) ,this time i changed a js file name, and problem goes off.

my partner dont have server credentials ,even I have changed the hosting server ,but problem is still there , I have checked all the code there is no such thing