Your watch is solar powered, so I think it isn't charged lately. At a low charge, the watch goes into saving mode by switching off the display (while keeping the time, date, etc.). To charge the watch leave it in a brightly lit spot for a day or two. That should be enough to make the watch function as normal again.
For more information, you can download the manual of your watch here (direct download link):
Timepieces Watches Manuals CASIO
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