There are two ways to close a deal: drag the card onto the Won or Lost zone at the bottom of the board, or use the buttons on the record itself.
Won
Won asks nothing further. The deal leaves the board, the date is stamped on it, and it counts towards everything that measures what you have sold.
Mark it won when the customer has committed, not when the money arrives. Those are two different questions and the product answers them separately: won is in reporting, paid is in Revenue.
Lost
Lost asks why, from a list your admin maintains under Settings, then Lost reasons. You can also type a reason that is not on the list.
That prompt is the most valuable thirty seconds in the product. A year of honest lost reasons tells you whether you are losing on price, on timing, on scope or to one particular competitor, and those four problems have completely different fixes. Without it you have a conversion rate and no idea what to do about it.
Keep the list short. Six reasons that get used beat twenty that get guessed at.
Where closed work goes
Closed deals come off the board but are never deleted. Find them with the show closed filter on the board or the list, in search, or on the customer's own record where their whole history sits together.
Reopening
Open the closed deal and move it back to any stage. It returns to the board with its history intact, including the fact that it was closed and reopened.
That is the right way to handle a customer who comes back six months later, and it is much better than creating a second deal and losing the story.