I accidentally deleted save files, put them back, but can't load game

Did you open the game before putting the files back?

Yes, I think I did. Thats when I found out they weren't working

AH that makes sense, maybe the game made a new folder with files in it. 

Could you check if you have any double save file names in the saves folder?

@Allie15678910

You can rename them to see if that helps the game recognize the saves again.

If you still have your saved games in your Sims 4\saves folder - [C:\Users\Username\Documents\Electronic Arts\The Sims 4\saves]

It may be the game isn't reading the save files correctly. To rename your save, remove the version number. - .ver0 or .ver1 by deleting the .ver1 on the end.

For example:
Slot_00000002.save.ver1

Rename it to:
Slot_00000002.save

You may want to give it a different number if you already have a save 2 in your folder. The game should then recognize the save file again.

-- If you have an old auto save file (Slot_00000001.save) it's safe to delete as the game doesn't use it anymore.

If the above doesn't help, Crinrict has some very useful tips on saved games disappearing:

Save Games disappearing
https://sims4.crinrict.com/en/2014/09/save-games-disappearing/

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When I've come across the problem of having a lot of saves in my folder and need to rename them is to move the saves folder to the desktop, and copy back only a few or so, maybe 10 at most at a time, and rename them in a 02.save, 03.save, 04.save, etc. up to 10.save

For example:

Slot_00000002.save
Slot_00000003.save
Slot_00000004.save

Load the game, select a save from the Load Game Menu, click the tiny folder under the name of the saved game. Use this information to keep track of the saved games.

If you're missing any families and you have more than one save, check Manage Households, it's up in the right corner of the map screen.

In the My Households tab (which is automatically selected) you should be able to find what families are in-game and what world they are in.

Please note, If Save is continually used, the game progress gained or family played may be lost. This is why it's important to use Save As, give the save a new name, save often and back the files up in case they maybe needed later.

Thank you so much! 
Turns out I had two folders with saves. I must have screwed up when trying to put the files back again. :-)

Thank you for your answer! It seemed to work, just deleting one of the 'saves' folders. 
But this is very useful, I will keep it in mind if this should happen again :-)