Arduino BT not found


I tried to connect the Arduino BT-Board via Bluetooth to my PC. First that worked great, everything was ok. But when I tried it from another PC, the board was not found. Now it is also not found on the PC I tried it first from.
The battery seems to be loaded, because a LED is glowing orange. I also tried to reset the board, which has completely no effect.

Has anybody any idea what is the problem here and how it can be solved?