Warrant for "Failure To Appear", what do I do?

You need to turn yourself in ASAP.  While the authorities will probably not actively pursue you for this, if you are stopped for any minor infraction, even jay walking, or if you have an employment background check run this will all turn up.  At that point you will be taken into custody.  And it will be worse for you to appear in court courtesy of the jail bus rather than coming in on your own.

Right now you need to consult with an attorney in the area where the warrant was issued.  They will best be able to use their local contacts in arranging for your surrender and help in minimizing the penalty imposed.  You will receive a sentence for both the FTA and the underlying charge (since you failed to complete the initial sentence for it).

It sounds like you were participating in a diversionary program, which, upon successful completion, would have resulted in your charges being dismissed.  When you failed to successfully complete it, due to the reasons that you listed above, a warrant was issued for your arrest for failure to appear on the date that it would have otherwise been dismissed.  Therefore, you now have to face the new charge (of failure to appear), and resolve your old charge.  I suggest that you consult with and hire a local attorney as soon as possible in the interest of resolving these matters as favorably as possible.  It is only a matter of time before you are picked up on the warrant, so you are better off taking care of this sooner rather than later.  Good luck.