Excel says file is locked
I have just migrated my old iMac files (Sierra) to a new iMac (High Sierra).
I find Excel will not open the Excel files. It says they are locked (by me, it says!) but I can open them Read-Only.
I have never locked an Excel file; I don't know how! Searching Excel help finds nothing for "lock" at all.
Please help!
iMac with Retina 5K display, macOS High Sierra (10.13.4), Calendar 10,Mail 11.3,Contacts 11.