According to the schematic I have for your mower, you have the tamper proof 4 prong seat switch which as the name implies can not be bypassed.
SO get your gun out , track down all of the idiot bastards who posted instructions for bypassing the seat switch all over Your tube & Face book and give them a big thank you right between their eyes.
Don't worry about hurting them because they had no functioning brain in the first place.
If you touched your switch. unplug it then replug it tight.
These switches have a shorting terminal in the plug to prevent you bypassing the switch
The switches themselves rarely give any problems but brainless morons seem desperate to sound like they actually have a brain by assigning every problem from a flat tyre to a con rod through the side of the block to the seat switch.
The simple test is to sit in the seat, turn the key to the on position then turn on the PTO switch.
You should hear the PTO go Klunk if the seat switch is working .
IT is highly unlikely that you would have disturbed the seat switch when changing the decks
However you may have disturbed the reverse switch which according to the schematic I have here will be on the tranny, or on the frame near the tranny so when pushing down on the revers pedal you depress the plunger on the switch.
These do get bumped around and will cause the engine to cut out with the PTO on & the brake off .
The schematic for your mower is up the front of the parts book and the parts book is a free download from Husqvarna