1: go into disk management and see if its listed there. If it is and no drive letter is listed rt click and assign it a drive letter.
2: try using it in another port (rear one preferred) or on another computer
3: try another cable
If it still cant be seen in disk management it could be that the usb to sata adapter inside the enclosure has failed. Often you can disassemble the enclosure and take the drive out and install it into your pc, another enclosure, or a drive dock and use that to recover files from
Please always remember that important files should never be trusted to just one device. Having multiple copies on different devices is the only way to survive a drive failure.