Most uploads that look stuck are still working or are waiting on something simple. Work through these checks in order.
Give a large file time
A long video on a slow connection takes a while, and the progress bar can sit at a high number for several minutes near the end while the file finishes transferring. If the percentage is still climbing, even slowly, let it run.
Check your connection
A brief drop in your internet connection can pause an upload. Confirm you are online, and avoid switching networks partway through. If your connection dropped, start the upload again.
Keep the tab open
Closing the browser tab or putting your computer to sleep stops an upload in progress. Keep the tab open and your computer awake until it finishes.
Try again
If an upload has clearly stalled, cancel it and start over. A fresh attempt clears most temporary problems.
Still stuck
If the same upload fails repeatedly on a good connection, the file itself may be the issue. Try a different video to see whether the problem follows the file or stays with your account, then contact support with what you found.