I just read the NTSB press release 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 (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.