Here we have a common problem when using Sylenth1 in FL Studio 11, 12 and 20. Every now and then Sylenth1 just quits making sound. This can be really frutrating, but as usual there's an easy fix.
First of all, make sure to have a legit version of Sylenth1 installed on your computer. If you want to buy it, go over to the LennarDigital offical website.
In most cases it has to do with your antivirus blocking some of the important Sylenth1 files. In order to fix this, you have to make an exception in your antivirus for the folder where you have Sylenth1 installed (or the .EXE file itself).
Your Sylenth1 installation folder can be located anywhere on your computer, but the default folder should be at C:\Program Files\Steinberg\VSTPlugins\Sylenth1.
Do you use Avast? Click here to see how you make exceptions for files and folders.
Do you use AVG antivirus? Click here for a tutorial on the same thing.
Do you use another antivirus? Simply Google "Your Antivirus make exception for file" and you should be golden.
You can also try to disable your antivirus all together just to check if it really is the cause of your problems. According to my research, it usually is.