Laptop keyboards are compact, lightweight, and surprisingly comfortable to type on, consideing their size. Unfortunately, they are also fragile and can be expensive to replace if they become damaged. Usually, the breakage or loosening of an often-used key, such as "Enter", would require replacement of the entire keyboard, but that option is not always available.
If you want to avoid such an expense, first, examine the damaged or loosened "Enter" key. There are normally a pair of plastic pieces underneath it on the keyboard. These plastic pieces are meant to hinge into one another and hold the key to the keyboard. If they are missing, or appear to be broken, you will need to purchase new ones. Searching eBay for "[name/make/model of laptop (key,keycap,clip]" should allow you to find and purchase a new pair of hinged keyboard clips. It is also possible to use the clips from another key, one you rarely or never use. A good candidate for a donor key is the "Right Mouse Button" key appearing on many laptops. This key has an image of a small box with horizontal bars on it. Many versions of the key also have a mouse pointer on the box. Carefully prying up that key will net you a pair of useable clips.
Affix the harvested clips to the keyboard at the spot beneath the missing "Enter" key. There should be four small areas on the keyboard where the clip can be attached. Push the clip into this slot with a direct and steady pressure. Once installed, the clips should move up and down easily. If they are difficult to move, try removing and reorienting them. It is possible to install the clips upside down or backwards, so make sure you have aligned them correctly before proceeding.
Press the "Enter" keycap down onto the clips. It should attach with a click under steady and direct pressure. One you hear the click, pull your finger off and try pressing down a few times. The key should move easily and not lean in any direction. If the key leans, try applying more steady pressure to get the other clip to attach.
Laptop keyboard parts do not require very much pressure to attach or remove, if you are straining, try a different angle and verify that all the parts are aligned.