Having not learned my lesson a few years ago (when the EXACT same thing happened when I attempted to fix the downstairs toilet) I offer the following advice:
When fixing the innards of a toilet, you can't simply replace the one thing that's broken (this time it was the ballcock. Last time it was the plunger.) The wear and tear that broke that one thing (be it the ballcock, the flusher, or the plunger) has also taken its toll on the other components.
So don't do what I did, to wit: go to the hardware store, buy a ballcock, attempt to take out the old ballcock, break the feed where the water comes in, go to the hardware store, buy a new feed where the water comes it, break the existing plunger while trying to get out the old ballcock, go to the hardware store, buy a new plunger, etc.
Next time, truly, I'll know better.
Or I'll just call a plumber.