The Sims 4 is an offline game and progress is saved in your local PC only. if you have backup of saved game from old PC, you can move it to the Sims 4 folder on new PC. Then repair the game.
Right click on The Sims 4 game icon and then click on "Repair"
If you didn't take backup from old PC, there is no way to retrieve old game saves and you need to start a new one. Though your online purchases in game(stuff packs and expansion packs) can still be downloaded in game.
