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. Identify both the video and audio codecs
Use the File Inspector rather than downloading a codec solely because a filename ends in MP4 or AVI. A video may decode correctly while its AAC, AC3 or DTS audio does not, or the reverse.
2. Match the fix to the application
Media Player Codec Pack installs components used by compatible playback paths. MPC-HC and other DirectShow players can use installed filters, depending on their configuration. The modern Windows Media Player app may instead require support through Windows or a suitable codec extension.
3. Install the required playback support
Choose the current official Standard installer for general playback in compatible players. Reopen the player and test the same file. A 32-bit application needs compatible 32-bit filters; a 64-bit application needs 64-bit filters.
4. Use drivers for hardware problems, not as a substitute decoder
A graphics driver can affect hardware-accelerated decoding. It will not add every missing software codec. If a suitable decoder is installed but accelerated playback fails, compare software decoding before updating the graphics driver.
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: