There is probably active wiring in the box behind the plate. If so, it must remain accessible.

Can you live with the plate and just paint it to match the ceiling? What if you decide in a future redecorating project that you want a ceiling light again?

If there is an attic there just pull the box into the attic and turn it so it is accessible from the attic. If no attic see post by billhart

The cover plates when painted are very unobtrusive, so don't worry about it.

Another thing is that you could potentially use the wires for installing some recessed lights in that area. If you can fish the wires to the recessed lights (depending on the joists) then you wouldn't necessarily need to keep the separate box.

Thank you everyone. There is active wiring behind the plate and is now being used to feed some recessed lights.

I have a ranch and everything is accessable from the attic.

I'll see if I can live with the plate and if not, relocating the live box in the attic may be the best option. Thank you, again.