suleman, Welcome to EP!
Folks here are in general happy to help with electronics questions; however, it is important that you do your part by having reasonable expectations and asking specific questions with salient details.
From your post it would appear that you purchased a Dell laptop charger that had been modified in an unspecified way (or perhaps simply salvaged for parts???) to operate in an unspecified manner from a 12vdc source. You have subsequently replaced a mosfet (presumably because the device was no longer performing the unspecified function).
If you want help with a circuit, you should provide a full schematic, and give a complete description of the intended purpose of the circuit and then ask specific questions.
Good Luck!
Fish