Hello, I have read a few threads here and people seem very friendly and informative, so I wonder if you could help me.

Briefly, I had my viva on friday (2 days ago) and it didn't go as well as hoped. I was stressed as most people usually are and didn't defend myself as well as I would have liked particularly at the start. I got major corrections/ R&R, I can't recall exactly what they said other than I have to write some sections again and there is a possibility of another viva. Irritatingly many of the criticisms were absent sections that one of my supervisors suggested I removed (I did). This supervisor has essentially left the university and does not reply to any of my communications. There really isn't anybody who can replace her knowledge is there anything I can do? The only person who might be able to would be the internal, who I believe isn't allowed to now right? With what we covered in the viva a high portion of the corrections will be in her sections and I am very nervous that I could not meet the expectation without assistance.

Additionally, if I have to undergo a second viva, is that it? If they still have problems with it I fail?

I feel a combination of personal disappointment and being let down by my supervisor, I really wanted this to be over and to be able to start my life. So any input would be appreciated.

You can't do much until you get the actual report telling you what changes you need to make. You say yourself that you're not sure how reliable your memory is about what they said (understandably - even when a viva goes well it is stressful), so try to give yourself a break until that comes through. The different range of outcomes that are now possible, should be in the PhD examination regulations, which will be somewhere on your university's website somewhere. They vary between universities, so it's best to check the rules for your institution. I think it's worth reading up on that, and looking to see whether your university has anything like a code of best practice for PhD supervision.
When you get your report, sit down with your remaining supervisor (I'm assuming from your post that you still have one just the more relevant one has left?) and go through it. If anything is unclear, then you are allowed usually to ask for clarification from the internal (this might be best done by your supervisor). You are right though that your internal can't get involved in the revision process. It might be that when you see it written down that it's less scary than you think. If there's a real problem about expertise, then be very upfront and ask how it's going to be plugged. It might be that there's a source of non-obvious help e.g. someone in a cognate discipline who uses that method or theory. To be honest, if your former supervisor has left, then the university can't really expect them to continue to work for free for them, but they do need to make sure you have adequate support. You might though have to press them on it. Good luck and I hope the situation feels less bleak when you see the report.

Hi Sloss and congratulations for getting that far into your PhD - it takes a bit more work and you will get there in the end. As someone who was asked to resubmit her thesis, I went through this myself (and resubmitted a couple of weeks ago). I went through sleepless nights of stress, and the like. I drove myself insane (literally).

First, I would like to remind people that R&R in PhDs in certainly not rare. My student union said that about 1 in 12 PhDs gets an R&R - and this number (they said) has increased in the last 3-4 years - at least in the UK). So, it does happen. Every university has a different system of progress. If you are awarded major corrections, then you will be given some time (if you are in the UK this is going to be from 6-12 months). If you are awarded a R&R it will be more (t depends on the university). Usually, major corrections come with some safety attached to them (you do the corrections and that's it). Your examiners can still fail you after a R&R as effectively your examination progress starts from zero. A new viva may or may not be required. But if you do all the corrections as requested (to the letter) and keep your examiners happy, I do not see why the examiners would fail you.

The problem in your case is the lack of supervisor. Your university is responsible to make sure that they provide you with the right person to supervise your corrections. They will probably do so.

So, take a breath, await for the examiners report, and start afresh. Don't give up please.