Verify the .MP4 video you are watching has the same-named subtitle file ending in .SRT saved in the same file folder. If you copied or moved the .mp4 video from your memory card, you must also copy/move the .srt subtitle file with it. If you renamed the video file, be sure you also renamed the .SRT subtitle file.
Also check the settings of your player, and verify subtitles are enabled. If you cannot enable subtitles, try a different player. VLC Media Player is known to be compatible with SRT subtitles on all operating systems. Also, this plug-in might help users with Windows® Media Player.