It is hard to know if lengthy silences and withdrawals by friends are because they are trying to dump you or because they are busy and a little neglectful. I especially grapple with this at birthdays and at Christmas time, often continuing to send cards or emails because I assume the best---that my friends are just busy.
But when I am always the initiator, does this mean I am refusing to accept that they don't really want to continue the friendship? Is it better to assume the best and send occasional missives? How does one know?
What a great question! You really get to the crux of the ambiguity of many friendships, which is something that everyone grapples with from time to time. Just as the beginning and ending of friendships are often fuzzy, it's sometimes hard to gauge what is going on with a friend mid-stream, particularly if you haven't spoken to her for a while.
People's lives change over time and sometimes the threads that connect them becomes frayed and weak. Thus, many relationships we thought would last forever turn out to be transient----and friends, even very close ones, slip in and out of our lives for a variety of reasons. Other friendships are continuous but change in intensity and frequency of contact.
As you suggest, lack of communication may mean any number of things: that the person is engaged with or overwhelmed with other people and/or responsibilities (e.g. work, family, etc); that the person needs more alone time for herself; or that the person is either purposely or unconsciously withdrawing from you.
Being a good friend entails being sensitive to the needs of another individual and to the natural vicissitudes of friendship. Unless you have a concrete reason to think otherwise, you should always assume that lack of communication has more to do with the other person than it does with you. Sending periodic emails, short notes or cards to acknowledge her birthday, or holiday greeting cards are thoughtful and non-obtrusive ways to tell the other person you're thinking of her.
But if there is no pick-up on the other end---for example, your friend never initiates or reciprocates after you reached out three or so times---it makes sense to check in more directly to find out what's going on. You can either call her, offer to get together, or send her an email asking if everything is okay in her life and between the two of you.
In most cases, your friend's response will allay your concerns. If she doesn't respond or answers in some vague way, allow some time to pass and try again. If there is still no positive response after that, it's safe to assume that your friend is withdrawing or at least needs a break and you need to accept that.
I hope this is helpful.