Hi guus,
I have to ask the obvious question ... is the unit plugged in? I don't ask this as an insult to you, but I once traveled 4 hours (one way) on an emergency organ repair call on a Saturday, only to find the someone had unplugged the organ ... it was still in the socket but wasn't making the full connection. I could have better spent that 10 hour day doing something more usefull.
There is the users manual, but it does not mention anything about how a "reset" can be performed. Just in case you don't have that manual, I've attached it below to this posting.
My other thought is that one of the buttons might be stuck in the depressed position. I would look carefully at all the buttons, and if one is stuck in the down position, very (repeat v e r y) carefully try to lift it up with a toothpick - do not use a screwdriver for boat building - bigger is not always better.
I would also check the back panel for a blown fuse, and or connection issues ... loose AC plug, external speaker wires (if used).
Was the piano turned on and playing when the "fiddling" occured? If so, something could have either shorted out or overloaded the power supply (too many buttons being pressed at the same time possibly) and the problem could be internal, which will require the aid of a service technician.
I would not attempt any kind of self-repair unless you are an experienced electronic technician and have expertise in this particular brand of instrument. You could do more harm than good. I know that obtaining the services of a qualified technician can be costly, but in the long run, well worth the money to avoid further frustration.
If you do venture into the interior of the instrument, for gods sake please unplug the damn thing first ... or you might wind up 6 feet below ground in an instant.
You may also write to Viscount via Email ... they might be able to assist in a quick reset remedy, if there is such a thing.
User's Manual:
View attachment Viscount Opera Digital Piano.pdf
By the way, Welcome to our forum
. We hope that you will become a regular participating member here.