Command to get items back

Unfortunately there is no way to retrieve your inventory whole as it was. You will have to replace each item yourself.

If you are playing singleplayer with cheats, you can enter creative mode to spawn and enchant netherite equipment with no material or experience cost. The command for this is /gamemode creative

If you are playing singleplayer without cheats, a workaround to the disabling of cheats is to open the singleplayer world to LAN using "Open to LAN" from the ESC menu. This will give you the option to enable cheats for as long as the world is open to LAN. Once you are done, you can exit and rejoin the world to make it behave like a singleplayer world again.

If you are playing multiplayer and you are the owner, you can either give yourself items through the server console, or give yourself operator status (using 'op username' or editing the ops.txt file) and switch your gamemode to creative.

If you are playing multiplayer and you are not the owner, you will have to contact the server administrator and ask them about retrieving your items.

Regardless of the circumstance, your complete inventory as it was is lost and you will have to individually spawn in replacement items. Note that in creative, you can apply unlimited enchantments to armor and tools so you will not be hindered by the prior work penalty.

As LexiTehGallade stated there is no way to retrieve your already lost items other than either just grinding out the effort to remake all your items or putting yourself in creative and remaking them.

For future if you are able to use commands you can set /gamerule keepinventory which will make it so your inventory is not wiped on death.