Yep, if you were saving to MP4, it's a pile of digital garbage now, sadly. Best to just delete it, and record to FLV in the future, as it tolerates crashes/truncations while only losing the last few seconds of video, rather than the whole thing.
There are tools to 'recover' MP4s, but they rely on a standardized encoding method to salvage a template 'atom' from the end of a complete MP4 and insert it into the damaged/truncated MP4. As OBS encodes differently each time, these tools do not work; about all that you'll get is intermittent clips of wrong-speed video and badly warped audio. Most of them are also for-pay only, quite expensive, and do not guarantee results (which you will not get with an OBS video, full-stop).
Better idea, if your Archive Broadcasts is set on Twitch, you can log into NightDev's TwitchDown site and auth with Twitch to grab a lost video still in the storage grace period from your own past broadcasts. I've had mixed results (a longer cast grabbing a corrupt chunk and failing the download, then having to wait 24 hours for the remote cache to self-clear) but it DOES work without exporting to YouTube.
https://www.nightdev.com/twitchdown/
