Website Codes gots replaced

Hello ,

Recently, I have bought a template and worked on the website. Bue to some plugin amendments, we had to give the website to outside developers to work on the website through FTP access.They successfully delivered the work and the work was stopped.

But just few days back the codes on the website got replaced to the old and majority of the changes made by the developers outside were restored back to the old version.

We contacted the developers to fix the issue. They were ready to help.

Now my management need to know the exact cause of the codes getting replaced by old ones and how is it even possible without anyone actually working on it.