Hi Kenzo
The solution suggested by Amala is the best, however, it requires you to review modifications possibly performed previously on the kinds of already existing material types and this may, in my opinion be long to restore.
First of all, it is necessary to identify on what environment the deletion was made. If in the development environment, the solution consisterai to open SPRO on production base (or the tests base, if you have one), make a minor change (for example, in the description of the item type) to generate a transport order that transfer you to the base of development.
B.R.