Can't Open File

Trying to open an xlsm file; now get an error message when I click to open it: "Excel cannot open the file because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."

I've used this file numerous times in the past, but recently have had some problems with retrieving files from my external storage.

How can I fix and/or retrieve this file? I tried the CNTRL-O remedy seen on other topics to fix corrupt files, but I can't click on the file without it showing this error message.

I'm using Office 365; have been using it with the file. My external hard drive on which the files were stored has been freezing up sporadically, so I copied the file from the hard drive to a flash drive. Did not change anything, just copied it over. I have tried copying the file to my computer's hard drive, but results in same message.

Will do that. Thanks for the info.