How do you open your inventory in Undertale?
Solution 1:
While playing Undertale, you'll want to use C or Ctrl to open your menu and Z or Enter to select Inventory.
When you start up the game, the title screen features an option menu where you can reconfigure your keys. (or test the game with a joystick/gamepad if you want to use something other than a keyboard.)
In your case, it sounds like the controls might have been reconfigured at some point.
Solution 2:
Also, if you are playing on Switch, hit "X."