What
said is correct. You somehow toggled off your "view items" which default key is Alt.
You have to go into settings and toggle it back on and set the key to Alt again.
It should be Options - GamePlay - GamePlay - Item Labal Display - Set to Toggle
If it's Toggled on, then you somehow changed the key for it. Just go into Options - Controls - find Show Items and change it back to Alt
If that all fails. Just go into Options and reset your entire game settings and that should fix it as a last resort, IF you haven't made any other serious key setting changes.