Impossible for anyone to say based on the info supplied so far but highly likely if the bucket stuck and you kept turning. But fear not, you can sort it.. To diagnose properly, first job is to free the bucket. You can do that with the engine in the bike but you will need to remove the relevant cam sitting above it. That's easy if it's the intake cam (you can leave the sprocket in place). More work if its the exhaust cam but no real drama. If the bucket is free (or you can free it) it should move a little against the spring when you apply firm pressure with both thumbs. If the bucket is very firmly stuck, you can weld (or get a mechanic to weld) a nut on top and then draw it out with a puller. In the UK a new bucket is about £50. Make sure you clean up the hole before you put the new one in to remove any burrs.
Once the bucket is free, next step would be to remove the relevant spark plug and look inside for damage with a borosope. If that looks OK, next step is to re-install the bucket, shim and cam then turn the engine by hand again. If it rotates freely with no bad noises, then do a compression test (plug the kit into the offending spark plug hole, remove the other plugs then turn the engine over using the starter motor with the throttle wide open for up to 15 secs). If you get reasonable compression, you could safely say that the valve and piston didn't collide. Happy days.
If none of the above works, then it's engine out, head off and replace the offending valve. The piston will probably be OK if you only caused the impact by hand turning - you can take a look once the head's off. Most likely you will have a bent valve stem. This would be enough to prevent any compression.
If you follow this link, you'll find videos on my Youtube channel showing you every step of the process on a 1980 CB750KZ which is very similar indeed to CB750F. This includes how to get the engine out. Do be careful about some of the videos on the internet, especially ones that show engine removal single-handed with a scaffolding pole. My method is safe. Hope that helps. Happy to answer any questions as you get into it.
CB750 videos
