A few points here from a Controllers point of view.
Firstly.as has been previously state a 50% percent loss of power controllers are taught should lead to a "Mayday" call (just typing the word makes my blood freeze!).
Secondly, as has been previously stated, a controllers first response to a "mayday" calll will be simply "Roger Mayday". this is not disinterest on the part of the controller but what is taught because it is understood that the cockpit workload has just gone through the roof.
Finally, what appear to be periods of silence on the R/T,are actually the controller phoning everyone around to clue them into the situation and the possible repurcussions.
Sometimes what pilots fail to realise about the job of ATC is that only about 30% of the job is talking on the R/T, the rest is co-ordination between different units and agencies. This percentage increases dramatically on the co-ordination side in the event of an emergency situation.
A classic situation for this is a (fingers crossed) incident at an airfield, where the controller may be too busy to talk to you because he is phoning the fire service, ambulances ,police, d+d and airfield operations. it is not to say that the incident has been trivialised, indeed it is to say that it is being treated with the upmost urgency.