My name is***** and I am sorry to hear your dog is so uncomfortable this evening. As you stated, she is in pain, but the question is what is the source of the pain?
Based on your description, I would suspect that the pain is in her back. Bassets are prone to blowing disks in their backs and the pain can come on very suddenly like this.
Here is a link to more information on disk disease in dogs:
For tonight, I would confine her the best you can. No running, no jumping, no stairs, etc. In the morning she needs to go to your regular veterinarian for a physical exam and pain medication.
If she is unable to walk or her back legs stop working, she needs to go to the local animal emergency room as soon as possible tonight.
Best regards, ***** ***** this answer helps you.
Feel free to contact me if you have any more questions.
Kansas State University,
College of Veterinary Medicine Class of 2001