It sure does sound like the clutch (or something else) is not engaging.
you mentioned the decompression lever.. That would appear to have absolutely nothing to do with the symptoms you describe.. This lever pulls a cable which opens a valve in the head and releases pressure. If that valve is completely open there's no compression so it won't run at all.. however, if the valve is partly open, the engin may run...
If the decompression lever failed at the same time the engine started giving you problems, it may not be a coincidence. I would think that examining that lever and cable and valve might point to the direction of the 'other' problem..
Describe what you mean by "the motor turns over".. Most time this phrase means that the engine actually starts and runs.