Wow! Im rather surprised no one has answered this question after being up for a week.
By failing physical training, I assume you mean one of 2 things: 1) you fail the physical training aspect of basic training or 2) you fail to pass the pt test.
I'll answer them both.
1. If you fail the physical training aspect of basic training, you are pretty much done. The drill sargeants will work with you in most cases to so can pass. Once they decide that you are not going to pass, then you will be administratively separated from Army or worse, recycled through basic training.
2. If you are in either AIT or you are in the "big army" and you fail a record pt test, you are automatically flagged and put on a physical training program hosted by one of the NCO's and then you are retested after a period of 90 days. If you fail the second pt test, the paperwork to chapter you out of the army will start. Your chain of command may or may not decide to give you another pt test. In my experience (not that I have failed a pt test because I have not), although they may start the paperwork to chapter you out of the army for failing to maintain physical fitness standards, your nco's will continue to work with you and if you pass a pt test, then the chapter process will be stopped. This is not a rule and its different in every company. But, if you are a good soldier and you try your hardest, then your nco's will probably work with you. A good nco will look at your failure as his or her failure and to any good nco, failure is not an option.
The easiest thing to do is simply stay in shape and avoid this issue. The army is tightening up its belt and the best way to weed out the ranks is to make the physical fitness standards harder. A little food for thought there.
I hope this is helpful.
I was in BCT and failed the APFT due to penumonia which is obviously uncrontrollable so they recycled me from week 9 to week 2. After losing all morale they ended up discharging me in which I didnt care at the time. I now regret all that happened and wish I would have stayed thru. I was young and dumb but such is life. I leave in a month or so for BCT cause I was let back into the army reserves only because its impossible to go active if your dos prior service. Just think failure is not an option and its completly up to you whether or not you pass or fail.