You may accidentally clicked on the wrong X symbol, thus closing that particular page.
Here an effective tip to fix this problem faster and easier.
Reopen Closed Sites
Let's assume you have closed three tabs and now you want to reopen the three pages again.
Here is how you do it:
1. Hold on to the Ctrl (cmd for Mac users) and Shift keys simultaneously (together).
2. Without letting go the two keys, us another finger to press the "T" key.
3. Instantly, the last website (third one) which you have closed will reappear.
3. To get the second last website, press on the "T" key again.
4. To get the first tab back, press the "T" key one more time.
5. Now you should get back all the three closed tabs back on top of your browser.