OLE DB or ODBC error: type mismatch (exception from HRESULT: 0x80020005 (DISP_E_TYPEMISMATCH))

Sorry to hear that.  You could open a ticket, but here are a few suggestions.

Run the refresh in PowerBI.com - it will fail, but the error reporting is better (? why ? I don't know) - so if you look at the dataset in the "schedule refresh" area, you will see the failure notification -  if you expand that, it may identify the column that is causing the problem.  Then you step through your query (and rebuild it if necessary).

If that error does not show, go to PowerBI desktop and get into the data view - then look at the fields list and start by inspecting any columns that are flagged, then look for items that are have date format. Hypotheis is that a new value showed up in your data that caused PowerBI to "change it's mind" about the TYPE - moving from Text or Date Type to "any" or "text". 

May need to rebuild those. 

I do not think this was a matter of someone changing the format in the source - so the cause of the problem is not known - one day it was working, the next it was not - so I would not advise going down that path as a first step - unless you control everything and have easy access.

Good luck.