Read the word on the card
The card says why. See what each word means, and whether the file is offline because its drive is not connected.
A very large file
A very large file can be too big to preview in 3D.
3MF and STEP on Free
HEXOstudios Free previews 12 unique 3MF files and 12 unique STEP files, for life. Pro previews unlimited 3MF and STEP files.
Still stuck
Tell us the file’s format and size, and what you were doing when the problem appeared.