I went to mikva, and when I got home I realized I forgot to cut my nails. What do I do?

If your nails were clean from any dirt under them (and usually they are), if you realized that night and the mikva is still open, you should go back to be tovel again without a bracha. But if you only realized the next day, or even that night but the mikva is closed and you can’t go back, then it is fine and the tevila is kosher.

ט”ז סי’ קצח ס”ק כא, ערוה”ש שם סעי’ מח