How can I fix it?
MY calculator won't turn off except after being left alone for a long period of time and the buttons aren't responding. Also it only displays 0=. and I've even tried restarting it many times.
Try replacing the battery and then resetting the calculator.
The EL-531X uses a LR-44 (or equivalent) battery
Here are two images that show what to do.
(click on images to enlarge for better viewing)