--- Could be you've toasted the MOSFET or the diode. To find out, disconnect the diode and measure its resistance in one direction and then the other using your meter. If you don't have a DIODE TEST function, then get 1.5V cell (battery) and a 100 ohm resistor and hook everything up like this:
+--[-BAT+]---[100R]---[DUT]---[DMM (20mA)]--+ | | +-------------------------------------------+If the DUT (device under test) is OK, when you have it in the circuit in one direction the meter will read about 8mA and when you reverse it the meter should read zero milliamps. It might show a few microamps; that's OK.
If you've popped the MOSFET, you can find out by connecting the gate to the source and measuring the resistance between the source and the drain. It should be very high with the drain connected to the + output of the ohmmeter, and it should look like a diode with the ohmmeter connected the other way around. If it reads like a low resistance both ways, it's gone.
You can also test the 555 by disconnecting everything from its output, setting your DMM to DC volts and measuring the output of the 555 as you turn the pot. It should be low at one end of the pot's rotation and high at the other.
Depending on your meter, though, you might have to do something like this:
555OUT>----[1N4148>]--+------+ ------------------+------+