Yeah thomas is right about stiff mortar crumbling, but adding too much water makes it weak and runny. It's a balance. The ratio depends on the job, but for basic block work I stick close to 3 parts sand to 1 part cement. Too much cement makes it crack easy. Find a mix that holds together but still spreads smooth. Smith.elliot, that ratio is a good place to start and then adjust for how wet your sand is.
Wait, your wall actually fell over? That's wild. A super thick mix can just sit there like a brick without grabbing anything. I've seen mortar that's too stiff just crumble out from between the blocks once it dries. Adding enough water so it spreads smooth is key, otherwise it's just fancy sand sitting in the joint.