Try pressing the pencil buttton when conecting it , maybe you can start it in safe mode , after that if it boot's, try to remove the App's you instaled , if after this still doesn't works , boot it again with the pencil button pressed and format it using *#7370# , but hook it up to the charger , or make shure you have plenty of baterry.
If you have a realy bad luck and all this does not work , you have to take it to a nokia dealer.

Samtech