A dent across the nail usually means some form or disruption of the nail growth area.Did you injure your nails recently?It can be due to injuries.It can also be due to Beau's lines that appear when growth at the area under your cuticle is interrupted.Third possibility is of psoriasis. The inflammatory
on the skin and can also affect the skin cells in the nails.
I sincerely advise you to consult a dermatologist as treatment modalities for all are different and so a confirmatory diagnosis has to be made first. Without examination confirmation of a diagnosis is tough.
I hope that helps. Please do keep me posted on how you are doing or if you have any additional doubts.Warm regards.