Keep asking me fill up the water tank even it is already full

Inside the water tank there is a magnetic float which is causing the problem. This float is moving always on the top of the water tank as you fill it with water and in reverse as the water level goes down then the float is moving down to bottom also. So as the magnetic float is moving up and down is giving the signal for the water level if it is full or empty.
So possibly now the float is stuck in the bottom and even if you have refill the tank this is still on the bottom of the tank. This can happen from many reasons. First could be that the plastic column that the float is moving is full of scale and it can not move up and down freely any more so you can clean it from scale by decaling it and even clean it with a wire brush.
A second possibility could be that float have a tiny hole and some water enter in the float and this make it to stay on the bottom of the tank. You can visually inspect it and if possible you can dry out the water and weld with a super glue the hole if possible, if it can not be fixed then you need a new float.
If you wand to fix it temporary until you find a new float you can a plastic clip or similar to secure it always on the top however you when the tank is empty it doesn't register so you would really have to keep an eye on the water level to avoid any other problems with the machine.
Good luck

This happened to my krups and I have a very simple solution. I emptied the tank and microwaved it for approximately 4 minutes. This dried out the float and it started working again.