We had a similar problem once but it was the web host (and they admitted it). They had our office blocked because evidently someone forgot their e-mail log-in password and kept trying to log in repeatedly. So they placed our IP address on their ban list and no one from our office could log in or even view the web site.
After a call or two they unblocked it, i reset that password and the problem was fixed.
Also ISP have taking a new tack with web sites that are critical of Telcos in general or that particular ISP. If you've been writing anything like that, or pro-union etc this may have something to do with it.
Please read:
St. Petersburg Web Design Blog » War Against Internet Starting Slowly
for more info on this touchy subject