It's probably not going to do a whole lot, because levothyroxine takes weeks to get into your system, so whatever you took today won't even be affecting your for a couple weeks.
I agree with the "be calm" but drinking more water isn't going to help anything.
It's only considered an overdose if you have adverse effects from it. If you begin getting heart palps, rapid heart rate, etc, you should get checked out.
For future reference you might take a calendar or sheet of paper and mark down each time you take a pill, then you won't have to wonder. Or get a 7-day "pill minder" and fill it each week, then you'll know each day whether or not you took your pill.