Dates automatically changed to currency?!

[LRiver]

Hi, have a very large spreadsheet with thousands of records and dozens of tabs. We use it everyday. Was working on it yesterday and when I clicked Save, it suddenly changed the dates into currency (£). Changing format back to date doesn't work as the date (day/mth/yr) are nonsense e.g. year is saying 1905 instead of 2017!



We use Dropbox so went to look at previous versions in the hope of restoring it but they've all got the currency instead of date too now - very strange!

Hope someone can help. Thanks.