If you are having issues connecting to a particular service, you can test using telnet to help isolate the problem.  For example, if you are having trouble sending email but you can browse different websites, start by opening a command prompt (Start > Run > Type cmd > Enter).  Then type 'telnet yourdomain.com 25' without quotes.  If you get to the next black screen, you are all clear and should start troubleshooting other possibilities.  If you are not able to connect, then your ISP has most likely blocked port 25 (SMTP) at which you will need to try and find alternative methods sending email. 

Here are some other common ports to test:

HTTP: 80
HTTPS (SSLs): 443
FTP: 21
SMTP: 25
POP3: 110
SQL: 1433