my Canon X340 no longer works on MacOS high sierra

According to this Apple document that printer is supported by High Sierra: Printer and scanner software for macOS High Sierra, Sierra, El Capitan, Yosemite, and Mavericks - Apple Support

So download and apply the following: Canon Printer Drivers v3.4 for OS X

The last driver updates from Canon for this Pixma mx340 printer were for El Capitan, and were released in Aug/Sept 2017. If you do not have these versions installed, do so. Then with the printer powered on, and in the System Preferences : Printers & Scanners panel, click on your displayed printer entry, and choose Reset printing system…

Add the printer back using the correct Canon driver entry. Try the printer again. There is no assurance that these older printer drivers will work in macOS High Sierra.