The more tabs you have open in Chrome, the higher the chances you'll accidentally click in the wrong place and close one you wanted to keep open. The good news is that Chrome, like most modern browsers, keeps both a history of all pages you have visited and a list of tabs you have recently closed. Which you use to reopen your tab mainly depends on how quickly you notice it's gone.
If the realization that you've closed the wrong tab comes immediately, the solution is simple -- press "Ctrl-Shift-T" to reopen the most recently closed tab. Pressing the shortcut again will reopen the second most recently closed tab, pressing it a third time will reopen the third most recently closed tab and so on. If you really want to, you can use this shortcut to reopen every single tab you've closed since launching Chrome.
After Closing a Few More
You've closed a bunch of tabs, only to realize a few minutes later that among them was one you wanted to keep open. Instead of reopening all of them through the keyboard shortcut, click the "Menu" button and hover over "Recent Tabs" to see a list your most recently closed tabs. Click an entry in the list to reopen the corresponding tab. As you reopen tabs, more closed tabs that you closed less recently take their place in the list.
A While Later
When you only realize you've accidentally closed a tab hours after the fact, digging through the recently closed menu for it would be a pointless waste of time. In this situation, the easiest way to recover the lost tab is to look through your browsing history. Press "Ctrl-H" to open Chrome's History tab. If you remember the title of the page, use the search field at the top to quickly track it down; if you don't, you'll have to scroll through the list until you find it. After you've located your lost tab, simply click on its entry in the list to reopen it.
Reopening closed tabs only work in normal Chrome windows. If you open an incognito window and accidentally close a tab within it, you will be unable to recover it, since incognito mode is designed to leave no trace of your browsing history. Furthermore, any data you have entered in tabs -- a Facebook comment you typed up but hadn't submitted yet, for example -- may no longer be there if you accidentally close them and then reopen them.