Cheap soldering flux causing QRE1113 circuit not working
It is called 'debugging' :-) 

You are lucky in that you have a scope so follow the signal flow through the design from input to output. At each point check the actual value against the expected value.
You should know the expected value having designed++ the circuit.
In your case there are only a few stages.

If you detect an anomaly, your scope signal should give you some idea what is wrong. Of course all this assume you have a at least some basic knowledge of electronics. 

++It may be that you just copied a design without knowing how it works. That is generally not a good idea for exactly the case you are in now: it is fine until it no longer works.



Post edit: always start with checking the power supplies, clocks and reset.