I agree with Gary, and would not normally reply in addition to what he has already said, but I am homing in on your term "a year or so ago". Most Casio boards in the CTK-4400 price range are covered by a one year warranty, so you should check your sales receipt to verify exactly when you bought it. In addition, even though the CTK-4400 is now discontinued, I found a couple of original sales adds where the retailer was offering it with their own optional (for extra fee) two year extended warranty, so again, check your sales receipt for your particular sales agreement. Beyond this, as Gary alluded, it would not be in your economic interest to seek repair, at your own expense, as just opening the case to perform diagnostic tests would most likely exceed the cost of a replacement board, and attempting to do so on your own would be meaningless without the proper digital test bench equipment.
So, regardless of all of the above, if you need to be practicing for an impending exam, you need to be spending your time and efforts to obtain a borrowed, or rented, or purchased replacement board ASAP. Sorry I don't have a more positive response.
Best of luck with this, and with your exam. Perhaps the current virus situation will result in a postponement that will buy you a little extra time to come up with a viable solution.
Regards,
Ted