All the airports I had downloaded UK2000 for are missing. The airport building and even the tracks on the runway are missing. I had them before but when I downloaded the EGKK UK2000 I must've done something wrong.

Check your Scenery Library to make sure they are still activated in the simulator.

 

Also, check your Scenery Settings in the simulator, to make sure that 'Scenery Complexity' is set to at least 'Normal'.  Below that, you are unlikely to see the buildings.

 

Failing that, the most common sense thing to do would be to try re-installing them.

 

Good luck.