Be certain you decrease citrus and acidic frequency. Do not eat or drink citrus over long periods of time and swish with plain water (no lemons or things in it) immediately following or even during consumption of the citrus. Use Sensodyne Essential Care, but remember that it is not tartar control, so brush longer (not harder and NO hard bristles). Do not eat ice.
It depends on how long it has been and what the radiograph shows. You must see a dentist to evaluate it to know for certain what is going on. Already loose usually leads to extraction.
Tooth-colored composite fillings and porcelain crowns ("caps", "fake teeth") do not whiten the same way as the teeth, so I would suggest you whiten to the desirable and tolerable (due to some sensitivity sometimes) shade, then proceed with the crown or other prostheses.