Instructions on Inserting/Removing your SIM can help with activation issues/errors, browser connection problems, and a blank/frozen screen.
Don't attempt to insert the SIM card while your device is powered on. Doing so may harm the SIM card and/or your device.
Carefully reposition the tray into the slot and push it gently back into the phone.
Check out these interactive demos (Pixel 3 or Pixel 3 XL) for more info.
Related Topic: Remove SIM Card