#1
The wampler slostortion has 2 LEDs, a blue one for the main distortion and a red one for the boost circuit. Since yesterday the blue LED no longer lights up, at most it flashes once if I press the footswitch really hard.

I opened it up and used my multimeter to test it, the blue led actually lights up, although the mm displays OL (open diode). For reference the red LED lights up and the mm shows 1.567 V.

What should I do to make the LED work as normal again? or the LED has to be replaced?
Every weekend,I go eat McDonald's near my music center.

Counter: Having here?
Me: No, there.

Then he would give me that funny look.
#2
Sounds like it could be a bad solder joint. Re-flow all the solder joints in the path of the LED with a bit of fresh solder and see if it fixes the problem.

By the way, you never use an ohmmeter or a diode test function on a live circuit. These functions supply their own current to the device under test and testing on a live circuit will throw off the reading or destroy the meter. You should also never use the ohms and diode test functions on a component that is still in-circuit. Components that are parallel to the device under test will throw off the readings.