From the photo, it appears that there is grooving of the nail plate of your right thumbnail, starting from the base. You may have a condition known as median canaliform dystrophy of Heller which presents with a midline ridge and groove affecting one or both thumb nails. As to the causes, some may be due to repeated injury (e.g. habitual pressing down on the affected area by the opposing thumb nail) or due to the use of oral drugs. However, the cause in many cases are simply not known.
The treatment of median canaliform dystrophy remains difficult. In most cases, simply avoid further trauma to the area, minimize wet-work, use gloves during wet-work, stop any oral drugs that may be the cause, protect the adjoining nailfold with a bland ointment such as white soft paraffin and there is a good chance that the nailplate will revert to being normal again after several months to years.
Hope this helps!