Sorry I thought it was worth a try and may have at least given you access to the site while your ISP fixed a dns problem. This shows it's not a dns issue and assuming nothing has changed on your pc, as you say, it looks like you'll just have to wait till your ISP gets back to you. If they call you back it might be worth mentioning that you cannot access the site by IP address too, so it's likely a routing issue. They should be able to reproduce the failure to access the site from a machine on their network, if not then it could be your pc doing something strange (do you have another pc / laptop you could connect up to your internet connection to test?).
Having worked at an ISP before and knowing the quality of many of the helpdesk staff I wish you good luck getting this sorted in any sort of reasonable timescale!!