Start with a very large flat blade screwdriver and compress the spring to see if the valve moves freely. slide the blade under the cam away from a lobe and put the tip on the spring cap. Pry up on the handle to compress the spring and open the valve. If the valve stem moves freely with the spring, you should be fine and just tossed a keeper and lash pad. If the stem stays and only the spring compresses, you might have a bent or stuck valve. If it moves freely, get yourself a KD3087 valve spring compressor to reinstall the keeper. To install the keeper, remove the spark plug, turn the motor over by hand until the piston just starts to come up and then stuff several feet of 3/8" rope into the spark plug hole, leaving the rest hanging out. Continue to turn the motor over by hand until the piston stops tight against the rope. At that point, you can use the KD3087 tool and compress the spring and install the keeper(s), lash pad, and rocker.
You might have had very loose valves or you might have floated the valve either with a missed shift, or by running the engine at too high an RPM.