It's the aerial part that runs up to the ignition lock and should go round the lock right at the top of the ignition lock cover.
What normally happens is that the ring of the aerial dislodges inside the cover and drops down, so it is unable to read the code from the key.
Take the two dome-nuts off and check under the cover. If it's still in place, check the aerial ring as it may have got water in and they start to corrode inside. Replace it and it should be fine.