I can't get an erection anymore and I am 16 years old, up until acouple of days ago I was getting it fine

Have you spoken to your father about this? You really should, and get in to see a doctor.

Many things can cause this. Stress, anxiety, and any other kind of emotional upset. Diet, exercise, (like if you recently started a weight lifting regimen) medications, vitamins,

Definitely talk to your parents and get in to a doctor to check it out.

Don't worry about this type of stuff. Conce on study only every thing will will be fine. This is normal.

What do I do to regain the ability?

Perhaps you are masterbating too much? Give it a rest for a couple of weeks.
Next was this with a girl or just by yourself. But sometimes you don't and sometimes it will not stay hard. That is just life, What happens is when it does not work right, you start worry, then the more you worry, the more problems you have.

Relax, let it rest and then it should be fine. If not, a medical exam would be needed

Perhaps you are masterbating too much? Give it a rest for a couple of weeks.
Next was this with a girl or just by yourself. But sometimes you don't and sometimes it will not stay hard. That is just life, What happens is when it does not work right, you start worry, then the more you worry, the more problems you have.

Relax, let it rest and then it should be fine. If not, a medical exam would be needed

Ive not hung out with a girl, so I'm wondering if I can't get a full erection because I'm not horny enough by myself