i deleted my sim by an accident

@PsychoGamingV2 You can try a few things...

• If you have a previous save of your current game save that you can roll back to (before they disappeared) this is likely your simplest option.
  
  Spoiler

  To return to a previous save:
  
  In-Game Method:
  Go to the Main Menu and choose 'Load Game'. Select a saved game, then hover over the small folder icon and click this icon.
  This will reveal your most recent saves on that 'Game Save'. Select the 'Recover' button on the version (see time stamp) that you prefer.
  
  Game Files Method:
  Or you can do this in the game files by following the directions here:
  https://help.ea.com/en-us/help/the-sims/the-sims-4/how-to-restore-a-previous-saved-game-in-the-sims-4/
• Do you have a different game save that has them placed in it? If so, you can move them together, alone, or with that household into your library, then place them into your new game. You can then move them to the new household, etc. If you do this and anyone gets placed with them from your library (in the same household), watch out for if any of the other household members are already in your new gamesave.*
  How to save your Sim/household to your library.
• You can check if there is a copy of these missing sims in your library. If you created them, there is normally a saved copy of them in your library.*

*Note that if you place these Sims from a different game save into your new gamesave, or from your library, these Sims will only enter the game with whatever skills, etc., that they had at the time they were saved to the library, and the only relationships they will have retained is with whoever is saved with them to the library within the same household.

@PsychoGamingV2 

In addition to what @SheriGR said, you can probably recover the save before you lost your sims.  From the Main Menu, use the Load option which will bring up the saves panel.  Select the 1st save and look on the right side where the play and delete buttons are.  You should see another button next to the delete button that looks like this:

It is the Recover Save button.  Just hit the button and another panel will pop up for you to choose which earlier versions of your save are available to recover.  There can be up to 5 earlier versions depending on often you save.  Select the first one and recover it.  Once that is done, load that save and see if your missing sims are there.  If not, do it again and choose the second save.  If the sims are not there again, then keep going until you've recovered all you can.  If you still haven't got the sims back, then you might have to try another save file.  

Hope this helps