Normally on the internet, you download files from a main server that's hosting the files. With IPFS, it doesn't matter if the files are stored on the main server or on the computer of someone else who has that file. However, if there is no main server and the only computer that was hosting it goes offline, then there is no way to access that file since it's not stored anywhere else. IPFS doesn't upload files anywhere when you add a file to your local IPFS node (since storing unlimited files for free would be very expensive), so when your computer goes offline there's no way to access that file since no one else has it.
The file needs to either be stored on your computer, the computer of someone who viewed your file and hasn't cleared their cache, or the computer of a file hosting provider like pinata.cloud or something.