I dont think its your chain or sprockets, shimano sprockets are meant to look worn and uneven to the untrained eye, they profile the teeth to let the chain move up and down the gears more easily
I think the pawls will have failed in the rear screw on freehub, that means the working parts that let you freewheel
If they stick or break you can loose forward drive, its not a part you can see BTW
If you are having problems with the bike its because you have outgrown it as a cyclist, I would imagine that you are now racking up more miles than it was ever meant to do, time for an upgrade I think
Edit - Looking at your original post, by "middle sprocket" do you mean the middle ring at the front of your bike? the chain ring?
Because if you ride everywhere in the middle ring you will wear it out, you can tell this when the teeth on it are shaped like a sharks fin, after a while they get so rounded that the chain will just roll off the teeth under pressure
It will be one of these two problems, my last point is valid one though, and I must say well done for racking up the miles needed to kill a chainset!