why did excel reformat half the data cells to DATE format?

the workbook uses tables exported from Access, and the data is used to prepare graphic representations of the data. i had saved the .xslm file, rebooted my computer then reopened the workbook – about half of the data colums were reformatted to DATE format, both in the tables/worksheets exported from Access and on additional sheets where i bring the data from those tables so it is in a usable format for preparing graphs.

i am presently reformatting each cell to the correct format (about 3 hours so far), but since i don’t know what caused it i don’t have any confidence that it won’t just do the same thing again.

any insight or assistance would be appreciated