Skipping my period. When will I be fertile?

I am usually super regular and dh an I didn't get a chance to bd at ALL last month (I was sick with bronchitis for 10+ days and we're on opposite work schedules). I had a three day period in September and was due to start again this past Thursday. It's Sunday night and nothing. I took an hpt and it was negative. How am I supposed to know when my fertile peak is now? I don't use opks but I just bought a thermometer.