I'm on probation for my first time & I just recentley failed a drug test. What could happen to me?

It would depend on the state that you live in and the initial crime.
Hopefully it would be some public service, but that would be up to the ruling judge.
You can go to jail too.
Make sure that you do not lie at any point in this issue. Be truthful about the drug use and what led to it and that you found that you were week and depressed at the time of use.
This could help.

Normally for one failed test, nothing really, most likely they may put you on a tighter probation, more drug tests and the such.

If your crime you are on probation for, was drugs, they may look closer