Rear derailleur bouncing up and down in easiest gear
That sounds like either the A or B tension spring on the derailleur broke. 

The A spring rotates the cage to tension the chain. The B spring rotates the entire derailleur around the mounting bolt. If you pop the chain off the chainring on the crank to slacken the chain you should be able to see that the derailleur is moving freely without spring tension. 

The solution is to replace the whole derailleur, as I don’t believe they can be disassembled to replace a spring.