theoretically the two year negotiation period can be prolonged if the 28 member states want it to be prolonged - my understanding is that there is some quibble over whether the EU commission and the EU Parliament also get a say, taking it to 30 parties, all of whom must agree - however the electoral cycle here, and the mood music we've been hearing from the EU structures and the other member states suggest that the likelyhood of that occuring is limited in the extreme.
so, theoretically the UK and EU could spend 18 months hammering out a deal which parliament then rejects and the UK government then goes back to the EU with a flea in its ear and says 'sorry, this won't swing, can we talk again...', but firstly theres no reason to suggest that the EU countries will be remotely interested in a second negotiation of terms, and secondly the above time constraint makes that a practical impossibility.
you have to consider, first and above all, that A50 was deliberately designed to make leaving as unattractive, and risky as possible for the leaving state, and to ensure that as many cards as possible were held by the remaining states. there are a number of reasons why, in the UK's situation, thats not nearly as true as some commentators think it is, but the process was not designed to be constructive and helpful.
the time contraint - the two years that really just about 18 months - means that on the two year aniversary of A50 being triggered the leaving state just 'falls out' of the EU. leaving the EU often gets presented as being like a divorce, and in many ways it is, but with a divorce you aren't divorced until the Judge says you're divorced, whereas with A50, you leave the EU on the date specified and it doesn't matter if the arrangements are 70% agreed or 100% agreed but not passed into EU law, you cease to be a member at 23.59.59 on the 2nd anniversary of submitting A50.