IRFZ48 N and FAN7392 MOSFET Switch
All the pins you have N/C can probably be left that way.
HIN &amp; SD (NOT SIN) have internal pulldowns - see data sheet.

You have probably destroyed the driver
or
The FET is gate-source short (as happens :-) ) or even dgs short.  

Remove the 100 Ohm (giving your components names is a very good idea). 
Test resistance of gate to ground.   Should be 10k. If 0 Ohms or &lt;&lt; 10k FET is dead.

With 100 Ohms still removed, toggle LIN (input).
LO should follow LIN.
If not, driver IC is dead.

Either replace FET and/or driver OR, as this is presumably a low switching rate circuit, you can build a simple cheap driver.

___________________________________________

Discrete driver:







simulate this circuit &ndash; Schematic created using CircuitLab

Vin high = Q1 on = Q2 on = FET on.
D1 zener voltage rating should be ABOVE Vdd = 12V so zener will not conduct due to supply alone.
D1 removes voltage spikes from Millar capacitance coupling from drain if load is inductive. [[ALL loads are inductive :-) !]].
D3 is a small Schottky diode mounted as close to FET bs pins as reasonably possible. On +ve drive cycles Schottky sleeps. If thgere are gate oscillations (as happen) schottky conducts on negative cycles and steals energy from oscillation.
Mount D1 as close to FET bs pins after D3 has been positioned. 

R1 probably wants to be more like 10 Ohms but I have used your existing value. R1 mainly serves to damp drive oscillations (which happen).
R4 may want to be more like 1k depending on switching cycle rate. A heat bed element does not need a fast switching rate due to the large thermal time constant. A 1 second rate would usually be VERY adequate and even much longer is liable to be fine.
R4 affects the FET turn off time due to RC filter formed with gate capacitance - usually in the 1 -10 nF range. (IN this case about 2 nF. T=RC = 10k x 2 nF =~~~ 20 uS so FET switches off over maybe 50-100 uS. Dissipation can be "high" over this period (maybe 100W+ surge) but as long as not done very often is probably OK. Smaller R4 helps switch off speed and does little harm. (This is of much less concern with your driver IC  due to active internal high/low drive. 

Make R4 say 1K.
Make R1 10 Ohm. 

Happy 3D printing - or is it T-shirts?



Faster switching:

For faster switching th FET gate capacitance must be charged and (especially) discharged more rapidly than the above circuit allows.

The following circuit is from Olin Lathrop and appears elsewhere on the SE EE site. This is a high side P FET driver - add the extra stage as in my above cct to get NFET low side - but I've posted this more for the Q14 + Q15 emitter followers which give high current gate drive. That pair of transistors can be added directly to my circuit. Olin's circuit has a ~= 200 nS switching speed. 

Carefully work through how Q2 R14 R15 work - it's 'trickier' than at first apparent.