When you boot the Amiga it will try to initialise the 3Dfx card in the Mediator. If it is not there, then it will not use it, and the Amiga's display will be used. That's what is happening now. When you connect the 3Dfx card to the Mediator it displays through that. From the sounds of it, you have selected a screemode that either your 3Dfx card or your TFT screen are unable to display. Follow Amigakit's instructions in the second post of this thread to reset your display settings back to their defaults. Then boot up with the 3Dfx card connected and select the resolution you were using before when it worked.
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moto