A .MOV file that won't play on Windows is almost always a codec issue — MOV is Apple's QuickTime container, and Windows often can't decode the video inside it (especially HEVC from iPhones). The quickest fix is to convert the MOV to MP4, which Windows plays natively.
Why MOV files won't play on Windows
MOV is Apple's format, and iPhones increasingly record video as HEVC (H.265) inside it. Windows Media Player and many Windows apps can't decode HEVC without extra — sometimes paid — codec support, so the file opens to a black screen, no audio, or an error.
The fix: convert MOV to MP4
Convert MOV to MP4 (H.264) and Windows plays it with no extra software. It runs in your browser, so your video isn't uploaded anywhere — which matters for personal footage.
Why not just install a codec pack?
You can (for example, Microsoft's HEVC extension), but third-party "codec packs" are a well-known malware vector, and you'd have to install one on every PC that opens the file. Converting once fixes the file itself, so it plays everywhere without extra installs.
What if it still won't play after converting?
If the MP4 also fails, the original file may be corrupted — an interrupted AirDrop or transfer, for instance. Re-export or re-send it from the source. For the general approach, see why won't my video or audio file play.
How to tell which codec your MOV uses
Right-click the file in Windows Explorer, open Properties, and look at the Details tab. Windows will usually name the video codec there — HEVC, H.265 or hev1 means you have found the culprit; H.264 or avc1 means the file should have played and something else is wrong.
There is a shortcut that is right most of the time: if the video came off an iPhone running iOS 11 or later with default settings, it is HEVC. Apple switched to HEVC to halve file sizes, and Windows never picked up a free decoder for it, which is the entire reason this problem is so common.
Stop the problem at the source
If you regularly move video from an iPhone to a Windows PC, change the recording format once and stop converting every time. On the iPhone, go to Settings → Camera → Formats and choose "Most Compatible." New recordings become H.264 in a MOV that Windows plays natively, at the cost of roughly double the file size.
There is a second setting worth knowing about. Under Settings → Photos, "Automatic" transfer converts video to a compatible format when you copy it to a PC, while "Keep Originals" hands over the HEVC file untouched. If your transfers arrive unplayable, that setting is often the reason.