Downloading a file in CRV format and saving as Excel

Hi,

I still used a much older Windows and Excel version and have had to very recently upgrade to MS 365 and Windows 10. 

Downloading a (ie) bank statement in CSV and filing it as an Excel spreadsheet was straightforward.

However, it is no longer so. I downloaded the statement and used Excel>Data>From Text/CSV - and filed it as an Excel file. When working with the data, I cannot use any of the normal formulas (ie vlookup, etc).

I assume that I must have missed something in the setup