Internal Server Error

[Arthur]

I've been using Shopsite Pro 11.2 hosted by hypermart.net for at least 5 years. Almost a week ago, my regular customers started calling to complain that they could not submit orders on my site. I tried and what happens is that after filling cart with items, filling out bill to:, ship to:, payment method then confirming order: when they press submit, they get white page with the following information.
INTERNAL SERVER ERROR
"The server encountered an internal error or misconfigation and was unable to complete your request.

Please contact the server administrator, support@hypermart-inc.com and inform them of the time the error
occurred, and anything that you might have done that may have caused the error.

More infomation about this error may be available in the server error log."

My site that normally produces 2-3 order per day, is dead.

I contacted hypermart (and even paying for premium service, usually 30 minute on hold), and almost a week later, they don't know how to fix it.
I was told that many other customers were affected by this ?? They said there was nothing if server error log.

I'm pretty desperate to get my site back up and running - if anyone has a suggestion I would be happy to hear it.

Also a possible different hosting company - I'm a 1000.00 in on hypermart but would gladly lose this to have a more knowledgable and better servers.

[loren_d_c]

Are you using UPS real-time shipping? If so, can you check the UPS settings in your ShopSite and see if you have the UPS AVS address verification enabled? If so, try turning it off and see if you can complete the order without that error.

-Loren