First and foremost, you need to find out whether she is still interested in being your girlfriend.
If you are the one who is initiating every conversation and always engaging her, this is not healthy.
As your girlfriend, she should be sharing in the responsibility of making this relationship work.
Hence, you should have an honest conversation with her and ask her about how she feels about the relationship.
She may actually no longer be interested in being your girlfriend. If that is the case, it is important to find that out now instead of hopelessly trying to keep the relationship going.
On the other hand, she may still want to be your girlfriend but something may be bothering her about the relationship.
This would be her opportunity to let you know what it is.
It may be an issue that can be worked on and resolved relatively quickly.
However, the first step is for you to talk to your girlfriend.
The sooner you find out why she is behaving the way she is, the sooner you will know where you stand and whether this relationship is salvageable.