Cannot open a video file after renaming it

It appears as if you have renamed the extension as well. You have not shared the exact information on which part of the file name have you changed. Please note that you cannot change a file format just by renaming it. You can rename the filename part, but not the extension. Check if you can add the original extension to the file and attempt playing it.

Henceforth, avoid renaming files including the extension. Some players would be able to read a file type even without extension, but most of them would need an extension.

Sometimes while renaming, the small characters get messed up, like the dot in between. As you said you can open the file in FLV player, you can copy the file from there, save it with the earlier format and try to play it again on the PC.
You didn't mention if the file was a media file downloaded from the internet, or if it was a personal video shot by you or any of your acquaintances, but in any case, if you can get the source of the file with the original format, it is better to download and keep the file with the original name and if you want to rename it, do it in the copy of it, so that if any error takes place, you can get back to the original file.