Welcome to Android Central! Which tablet do you have? Have you been experiencing problems with downloading or installing apps?
Go to Settings>Apps>All or Settings>Apps>Disabled (if this category is available), and look for Download Manager. Tap it, and see if there is an "Enable" button. If so, it somehow got disabled. Tap Enable, and it should work again.
If you don't have a "Disabled" list, then go to Settings>Apps>All, and scroll all the way to the bottom to find disabled apps.