Once an Investment is closed, it is no longer visible in the menu on the left. You can find all your closed investments in your Holdings table and if you click on any of them you will find a "Reopen Holding" button. If you click on it, you can activate the investment once again with a new, updated Valuation.
The only Holdings for which a "Reopen Holding" button is not available are Holdings with automatic price updates. These are automatically reopened when you add a new Position to the Holding.