The file attributes may have been set to read only. The old dos command 'attrib filename.xxx -r' may work in a dos window. The use of wildcards is supported for changing multiple files e.g. attrib file????.* -r' will remove the read only flag from all files with an eight letter name starting with the letters 'file'.
If all of this is nonsense to you, then leave well alone and work through file manager or whatever else you can find that works for you.