well, as i've mentioned on other posts of yours, there's a difference between real witches and movie witches, so don't expect sparks to fly from our fingers, or levitation or anything 'cool'.
'power' comes from outside ourselves. with practice you can get better with working with energy and casting spells, but you never 'lose your power', when your ability to cast gets weaker it's because you're out of practice not that it was taken away forever.
a witch can cast over another witch [for good or bad] but typically you can tell. also very rarely if ever is a spell permanent and cannot be reversed or stopped by the witch effected by the casting. if you were cursed for example, you'll most likely feel a heavy energy, you might also feel nervous or uneasy in some way, you can then cleanse and protect [perhaps if you know who's responsible you could try reversing the energy back on them] and you're fine. the majority of spells can be stopped if you know what you're doing.
is there a particular path you are interested in? you would benefit greatly from reading some books and articles on the topic of magick.
the above link should give you a basic idea of magick.