The good news: There is a lot you can do to reinforce that your PTSD is not her fault.
The following is based on my personal experience:
I'm a Failure: many spouses feel like a failure if they can't snap you out of it. They feel that your emotional distance is because they have done something to make you love them less. We all know this is not the case. Logically, they know that it's not their fault, but their heart is telling them something different.
Recommendation: Even though you don't feel up to it, force yourself to reassure her. Touch her on the arm, give her hugs, back rubs, your undivided attention for just a second to look her in the eyes and tell her it's NOT HER FAULT. Make sure it registers. Say it as often as needed.
Feeling Isolated: Look into finding her a support group for spouses who are struggling. In many cases, just knowing she's not the only one going through this can validate her feelings and make it easier to accept that the PTSD is the problem - not her.
Recommendation: A really good resource for this can be found here if you prefer face-to-face support. There are a lot of online peer-to-peer support networks. Check out this website for more information. You need to help your spouse find the support she needs. It can be hard to focus on anything other than yourself - PTSD is inherently a selfish malady from my experience (selfish, as in you get wrapped up in yourself and have not time for anyone else).
Feeling Overwhelmed: Your spouse, when things get this bad, often has to do everything alone - tending the house, working, tending to children, paying bills, etc. Oh yeah, and tending to YOU.
Recommendation: Take all of that responsibility for a while. As exhausting as it will be for you to tend to everything for a day, tell your wife to go take a day for herself. You take care of everything at home. If you don't trust yourself to attend to the children and you have family nearby, tell them the situation and ask them to take the children off your hands to help your spouse stay sane. If you are feeling stable enough to reciprocate emotionally, take a day away for both of you. Your gentle and intimate touch can make all the difference in the world.
These are the big ones I can think of. There are other marital issues, but concerning reassuring your spouse that your PTSD is not her fault is pretty well covered by this list. If there is anything I didn't cover or any specific questions, please comment on this blog post. I would also love to hear from other people to hear what they do to battle this.