Check the file size first. Free tools cap at 100 MB; AI tools at 100 MB; Image to PDF at 20 MB per image.
If the size is fine, try a different browser. Some privacy extensions block file picker access; an incognito window with extensions off is the fastest way to confirm.
If the upload starts and then stalls, your network may be silently dropping the connection. Switch to a wired connection or a different network and try again. Free tools never actually upload (they read locally), so a stall there means the file picker itself failed — refresh the page.