Of course you could be pregnant if the Mirena has come out. How would it work if it wasn't in there? And estimating your chances means nothing. You are or you aren't. Stop having unprotected sex until you know, if you don't want to be pregnant.
The thing to do now is to see your doctor to figure out if the Mirena is still in there (it can move up or the strings could be up inside your cervix), or if it's out. Or if you are pregnant or not.
I had the Mirena and got PID so I had it taken out. Now my doctor is suggesting I get it back for endometriosis. But I am having second thoughts. My mental state was horrible when I had it and I was overwhelmed with worry as to wether it was in place or not. I've been reading y'all comments and I am not going to do it. I've been reminded of how it really after two years started messing with me.