St170 Problem!

Hi, I am wondering if anyone here can help me or give me some advice? I have got a ST170 which gets a full yearly service and is a nicely looked after car but yesterday on the way home from work the car stalled at some traffic lights and after some turns of the key the car fired up again but on the drive home the car wouldn't rev over 2000 rpm. I knew from personal experance that the car wasn't in "limp home mode" as I have had it before when the clutch went on the car and it just felt to me that the spark plugs wasn't working, which I thought was strange but with the service comming up in April I thought that perhaps one had gone.

Today I checked the spark plugs and number one had oil in it's port - sorry if at any point I get the termanolgy wrong, I'm a maintenance fitter but defo not a mechanic ha ha - so I took the rocker cover off, cleaned it all up and wire brushed the spark plugs, which still looked brand new but the gap was a bit larger than the manuls recomend gap of 1.3mm then resealed the rocker cover gasket. I then started the car and it ran nicely for 10 mins so I went in washed up to get ready to take the car for a test drive but when I started the car up I heard a bang and the inlet maniford had blown off!! After lots of swearing I managed to get the maniford back on but the car still won't start it just judders but won't fire up. I have checked all the spark plugs are sparking on every ht lead and everything else my limited skills on a car will allow but I just can't get it to do anything!

So my question is why did the inlet maniford blow off and why doesn't the car start after it ran perfectly after I did the rocker cover gasket? Hopefully someone can give me some advice and I can repair it myself because I get the feeling getting the car towed and repaired at a garage will cost me a fortune!

Cheers

Dan