How to make girlfriend happy when she is not picking up my calls

Send her flowers?

Depends if you have had a row maybe just let the dust settle,if she is just not picking up with no reason that you can understand,perhaps go to her..

But be calm,there may be a reasonable explanation,listen to what she has to say.

DON'T Overreact! If she has a good reason for not answering and you find out later, if you acted like a jerk in the meantime you will seriously regret it. Otherwise, I would just wait for her to contact you, if she's mad she will come around on her own time and if something serious happened, she will tell you soon enough.

Chill out, don't try calling her too much because then you seem annoying! Leave her a message (if she has voicemail) Or just wait for her to call you back! I'm sure your fine

Do not call her. That would make her happy, not having to screen you out. Leave her alone.