I would convert the video file to MPEG-4.

 

Are you using the stock player, or have you installed a 3rd Party media player? Are you also recording using the stock app or an app you've downloaded from the market?

You shouldn't have to convert files that you've recorded on the phone itself (unless you're using an app you've installed yourself and the stock player doesn't like the format).

If you transfer the file to a PC, will it play on that?

 

Hmmmm It may be a corrupted file somewhere in your phones OS

 

Are you recording at 720p? A quick google shows some issues recording at 720p using the standard Class 2 SD Card that comes with the phone. I take it you haven't replaced the SD Card at all?

Also, there are apparently issues with video playback on Froyo, if you go Settings -> About Phone -> Software Information, is your Android version start with 2.2 or 2.3?

Try a software update: Settings -> About Phone -> Software Updates Settings -> About Phone -> Check Now (that's what it is on my handset, it might differ slightly on yours).

 

What class is your Micro SD card?

 

Well this is strange , All I can think of is backup all your stuff and do a factory reset.

 

Look at the card, it should tell you the size, then have a number in brackets after that. That's the class. It denotes the read/write speed of the card. I've had issues with class 2 on my handset when recording HD video, generally anything above Class 6 should be fine.

It's temperamental by the looks of things, it might not happen every time. Is there any way you can try another card?

 

If you've got a different card, I'd try that. Did you try for the software update too?

 

I'd say the video you made is gone for good unfortunately.

There are instructions for doing the update a few posts up (just click here).

 

I have experienced this same phenomenon with the Droid X where some videos will not play back. I make lots of videos that come out fine. I have a strong hunch that the reason this happens is that there are a few seconds after you stop recording before you get your menu back. If you hit the back button after hitting stop on your video but before it can finish its end of recording process you will get a video that is unplayable. Its like it is finalizing the video or writing header records or something. Just a hunch but I believe that is it. If you still have the video it could be repairable with 3rd party video editors but not sure.