If you have older mdb databases, especially created by older than Access 2007 (I think it was 2007), they WILL NOT open in Access 2016. I've seen that issue recently.
Here's where I asked about that:
thread181-1775799
From Microsoft, they just say use an older copy of Access to convert the database to accdb format. Several other online forum postings show other having the same issue.
I suggest seriously sifting through your Access databases, and upgrade all mdb files where possible so you have no chance of hitting the issue after upgrading. I had no issues with Access 2010 or 2013, best I recall.
"But thanks be to God, which giveth us the victory through our Lord Jesus Christ." 1 Corinthians 15:57