Supporting Mac

Hello all, I am new here, but recently purchased a used Macbook Pro laptop, which is giving me issues using my external hard drive....

I had an external hard drive that was connected to my new laptop for about 2 hours, and then out of nowhere it just randomly unmounted the drive, and now it will not reconnect. When I plug it into the USB port, the external drive lights up, and so it is connected, but the laptop will not recognize it. I have tried rebooting the computer, and checked in Disk Utility to see that the drive is there, but it is unmounted, and clicking on Mount does not work, an comes back with an error message??

This just happened randomly for now reason, and I do not believe there is anything wrong with the external drive, not the connectors, so it must be something the laptop is doing. Can someone explain this to me please, and Thanks in advance!!