Your motor needs three things in order to start and run:

1. Spark, easily checked by using a spark checker to replace spark plug.

2. Fuel

3. Compression, lack of this would signal serious problems, but I doubt this happened.

If I had to guess, because the car died suddenly and the engine still turns, you lack either spark or fuel.

One thing to check, since you mentioned a hissing sound, is for a large vacumm leak, the plastic line the supplies vacumm to the brake booster can break and result in a large enough vacumm leak so that the car will not start due to a leaned out fuel mixture. The knocking sound may have been detonation caused by the lean fuel mixture. The line takes a few minutes to change and costs around $50.00 or so. Most MB dealers stock the line or can get it overnight, you will need your vin number to order it.

Make sure you put a dollar limit on the work order before you sign it, say for a couple of hundred. That way the mechanic doesn't have a blank check for repairs without your approval.