I just received a call today, but my phone locked up during the call, and when I went to the missed call list, the number didn't show up. Is there any way to somehow recover the number that was calling?
Your network provider may provide a "Last Number Redial/Called" or "Call Alert" service.
This is usually a number that you call that will tell you the phone number of the last incoming call to your phone, or can often be implemented as a service that you can turn on and off (again, usually by calling a number or issuing a USSD code) which will inform you in some way (usually by text message) of missed calls.
For example, in the UK, O2 used to have a number, 1471, that you could call to hear an automated voice message telling you the number of the last person that called your phone. Unfortunately, they have withdrawn that service however, they have replaced it with a free "Call Alert" text message service.
Note that there is no universal mechanism to retrieve the number of the last person that called you, if you no longer have (or never had) that information saved within your phones own call log.
Each of these services is specific to your mobile network provider who may or may not even offer such a thing. If the last number that called you is truly missing from your phones own call log, you may try to get in contact with your network provider to see if they offer a service as described here. Also note, though, that even if they do offer such a service, it'll usually only retain the very last number that called, so if someone else calls you in the meantime, the number that you're seeking will probably be lost.