Problem Srt not showing on screen

I have a Samsung UBD-K8500 Ultra HD Blu-ray player.
(In settings/sysrem/language...the subtitle is set to "on")
I have a portable hard drive plugged into the USB port.
I play all my videos from the portable hard drive.
My videos are mostly mkv and mp4s. I have .srt subtitles for all my videos.
Yesterday, everything was working perfectly.
But, now, the subtitles are being displayed on the screen.
I have watched these same videos before and all the subtitles were on the screen.
Does anyone know why the .srt subtitles are suddenly not being displayed on the screen?
I would appreciate any help.
Thank you