There is no automated way to "upgrade to Magento 2". So this can only have been done by someone overriding the files with those from a Magento 2 package rather than Magento 1 package.
What work has been conducted recently? Do you make changes directly on the server or do you have a development company that do this for you?
If the files have been overwritten, then you will either need to get a file backup from your host or from your own backups. The database is unlikely to be affected at this stage but it's hard to say.
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