I just read the [NTSB press release](http://www.ntsb.gov/news/2010/100318.html) of Northwest Airlines Flight 188 that flew more than one hour without making any radio contact. I imagine there may be many causes for losing radio contact with the ground (distraction, electrical failure,...) and I found a [video](http://www.aeronewstv.com/fr/article/1790-comment-se-deroule-une-interception-en-vol.html) (in French) saying that one way to solve the situation is to send a fighter to have a closer view of the situation and possibly assist the pilots.

I wonder if the situation of losing contact with an airplane is frequent? I imagine it happens between one and ten times per month over a region as big as the USA, but that is just a guess.