Question: I am no longer able to start my 2010 Ford Focus. The key is not able to turn at all. 

Importantly, the car beeps when I open the driver side door as if the key was still in the ignition. This has happened before, but I would simply reinsert and remove the key. Then I would feel/hear something mechanically click into place and the beeping would stop. The problem is I can't make the click happen this time.

Any suggestions? What part will need to be replaced?

Answer: 
Could be your key is extremely worn or the ignition lock cylinder is extremely worn. I suggest calling a local locksmith to replace key and code a new lock cylinder to oem biting for your car. Ford ignition locks are common failures.

You could also try flushing it out with some wd40 but it would be a temporary fix at best.
Often the dust cover fails to close leaving the key hole exposed and this causes the issue described.

Sometimes a **tiny** bit of lubrication is sufficient (clean first) and then the cover closes smartly.