@Shinea so sorry for your loss, and that you're still waiting for your period to return.
As other ladies have already said, it's a bit of a lottery as to when it will come back - have you taken any HPTs to see that your HCG has returned to zero (or rather, undetectable by a test), and if so, are you using OPKs to see if you are ovulating? It's just one way that you can try and see how things are going as you won't ovulate until your HCG has returned to a minimum point, and without ovulation you won't have a true period (vs. an anovulatory cycle for example), so the two are connected.
About when it's safe to try again, really this should be dictated by your physical and emotional recovery. I'm just repeating the good advice you've already been given, but this will naturally vary person to person. More recent studies point to that there is no reason to delay TTC provided you have not been explicitly told because of medical reasons that you should wait, but some doctors will still advise to wait one cycle, or even three before you start again. There is plenty of information online that can help you make an informed choice, and if you have any concerns, obviously seek reassurance from your doctor.
As to specific reasons why you sadly had to suffer this, only your doctor will be able to give you a steer (did they take anything for testing?) but the Miscarriage Association has page and leaflet on late MC, which might give you some more general information for now.
I hope this helps a little xx