Sometimes the file system on the card gets corrupted. It can happen for example if the card was ejected while a file operation on it was in progress. When that happens you might have trouble accessing some of the files, loading times for some directories in file explorer may be very slow.
You can attempt to fix it using the desktop windows' built-in chkdsk utility.
Open up command prompt (start->run->cmd.exe) and type
chkdsk /X /F <SD card drive letter>
My SD card reader is X:\ drive, so i would use the following command:
chkdsk /X /F X:
SD card reader is recommended of course