How to appologise to my friend

Lately my dear friend has gone through a lot and has not spoken to me for a while, I became very nervous and found out where she was at and decided to surprise her. After a failed attempt she found out and was very upset and I feel extreme guilt for it. A few days have passed and she still only reads my texts and dosnt answer. How long should i wait and what's the best way to organize my feelings to a deep apology that she'll see how bad I feel about it. Please any and all help is appreciated she's my best friend and about the only one I got