You do not need to remember the complete code
“0x something” describes a partly remembered Windows error; it is not a specific error code. Several different problems can produce similar wording. Start with the symptom you can recognise, then use the checks below to narrow down the cause.
1. Separate the container from its contents
MP4, MKV, MOV and AVI are containers. A player must understand the container and decode the streams inside it. The File Inspector can help you distinguish an unsupported codec from an unfamiliar filename.
2. Try suitable codec and container support
For a complete, unprotected file, install suitable playback components or try a player that supports its container and codecs. Media Player Codec Pack can improve support in compatible Windows players; it cannot make every format work in every app.
3. Do not rename the extension to convert a file
Renaming a file from MKV to MP4 does not change its streams. If you need broad compatibility, convert a playable source to MP4 with H.264 video and AAC audio. Keep the original until the converted copy has been checked.
4. Rule out other reasons for rejection
An unfinished download, cloud placeholder, corrupt container or DRM-protected file can also be rejected. Copy the full file locally and compare another player. Driver troubleshooting is more relevant when supported files open but decoding or output fails.
Which player showed the error?
Windows Media Player Legacy, the modern Windows Media Player app and MPC-HC use different playback paths. Installed DirectShow filters help compatible players that select them. Some Windows apps use Media Foundation instead, while other players use their own internal decoders. Choose the fix for the application that actually failed.
Does any of this wording look familiar?
These guides may help if you recognise more of the message: