Help!!! My site has turned in to a giant 404 error!

It was working fine last week...no problems. When I logged in yesterday, I discovered that I could not add new content, my permalink structure is different, along with a host of other problems. When attempting to load a page/post, I get a 404 error....on all of them. Please someone, I need help. I have no clue what happened. So far I have tried disabling all plugins, I have switched to default theme, I have resaved my permalink settings. None of these have solved the problem.

I am a complete wreck at this point. At this rate, I won't be able to achieve any sort of reliability for my customers and prospects. If my site is broken, I am out of business. Someone please help me get through this....