Easy things to try are swap out the USB cable.
Try it on another computer.
After that then you need probably to tell us which IDE and boards version are you using ?
Were you on your own computer in the lab or a school computer ?
Is it a real Arduino or a clone ?
And anything else you think may help people to help you.
Oh and go back to the top of the troubleshooting forum and read the posts from the top down !