You can go to a 20min morning nap and see if that helps her settle? We went down to 20mins for some time, and then a few weeks of morning nap refusal/10min naps before (I think!) finally dropping to one nap - ie we've stopped offering the morning nap at all as of this week, so all very recent! Also just to warn you, 18mo is a classic sleep regression, we've certainly seen this in our DD, she's been having more troubles settling, especially with travelling over Christmas, but on one nap now she crashes more readily!
So, personally, I 'd start by trying a 20min morning nap, if she still refuses a pm nap 3-3.5h after that then you could try 10mins. If that's a non starter, then you could try some one nap days, it's possible that she'll still need some two nap days to catch up though. How long was she napping in the afternoon when she did take that nap? That will give us some idea of what her overall sleep needs are in 24h, which will help in figuring out a one nap routine. But if she can get that pm nap back again with a shorter am nap, that will start to bring the pm nap earlier and longer and hopefully then when she does move to one nap, it will feel like a smaller change to the routine overall, if that makes sense?