I'll bet something in your correspondence has given her a bad feeling about you. Maybe that you're too clingy? Just let it ride. She has people in her life who care about her so if something bad has befallen her, people have noticed. Most likely, she's just busy or has decided you are not her speed. It's just life.

I say, be concerned if you feel you want to be but I wouldn't exhaust too much mental energy on it, especially  in regards to being concerned for "her safety".

I must insert this, I think it is a natural and first response for a man to immediately be concerned for a woman's safety in situations like this, however it's highly unlikely there is reason to worry for that.  

But on to my main reason for replying to this post..........I doubt if the PM system on that site is her PRIMARY and ONLY source of communication, and sometimes a lady will prioritize her time in online communication....and puts more of her energy into her regular work email  (aka forgetting to check all the various sites one may be on for pm)  yes, it happens.  

How 'bout this for a suggestion:  Send her ONE regular note to her regular work email, mention that today is Wednesday and apparently getting together today was not going to work out for her as she had not confirmed it with you.  Express your ongoing interest (if you have that) in meeting at a future time when both of your schedules permit.....  and leave it at that.  Keep it gentle and kind-spirited, of course.   If she writes back, great.  If she never does....well, you know what to do then.    

Keep in mind:
1) Most ladies do not have "help" with their emails and communication tasks....and it can become a bit daunting at times.  At least it did for me "pre-assistant" days.  
2) Some ladies, rather than a polite note telling you they have changed their mind about seeing you, OR that the day isn't going to work out after all, just simply do not write back.  

why do women do this?   Here's a couple of "my" insights to the oh-so-complicated female mind

   a) It's less work for them (why should they spend MORE time on you if they don't want to  
          see you in the future)  
   b) They assume that you, the client, will just KNOW what "no response" means
   c) they forget to do it

In any event, I hope you hear back from her!!
Portia

There are a million reasons she may not have responded.  She may have gotten sick.  She may have suddenly gotten called out of town.  Friends are visiting.  She may have decided to shut off all of her electronic communications for a couple of days.  Or she may have simply decided she doesn't want to communicate with you (or anyone else) right now.

Leave the situation alone.  Don't cross over that line.

You question is noble, but I'm of the opinion that ladies don't need hobbiests to look out for them.  

Of course, opinions are like ass holes...

Providers flake on communications all the time, even the good ones. You're panicking because it's been one day since she logged onto her site and emailed you. Don't be such a stalker.

But not much for you to do unless you have a much closer relationship than just a potential client.

Let it go.  She will get back to you or not, if not just find someone else.  Hobbyists don't always get back to providers which they cannot really do anything about either.  Life sometimes gets in the way of any plans we all try to make.