whether there is a quick solution depends on how long you have been using the laser. I am also a new proud owner of the Rotrics Dexarm and also own the 2.5W laser. For example, I noticed that there is no additional fan to prevent the smoke from the laser from getting into the lens.
This could possibly be the problem that your lens is clogged by the smoke from the laser or even worse the smoke is burned into the lens.
Possibly that is why the laser no longer shines.
It can also be that the laser has reached its operating hours and is used up.
I think it’s important to either put a separate fan in front of the object to be lasered, or to use a solder fume extraction system like the one here (“https://www.amazon.de/Minadax-Rauchabsauger-Abzugsventilator-Aktivkohlefilter-antistatischem/dp/B07S8CHB1V/ref=sr_1_4?__mk_de_DE=ÅMÅŽÕÑ&dchild=1&keywords=Lötrauchabsaugung&qid=1615008835&sr=8-4”), which at least prevents the smoke from getting into the lens during lasering!
I think times that increases the life of the laser also by some time!