(2-part post):
The free version of AVG (which is currently AVG Anti-Virus Free Edition 9.0) is not a firewall. It's simply a program which identifies and blocks computer viruses.
The paid-for version of AVG (which is currently AVG Internet Security 9.0) includes a firewall (which should automatically disable the firewall which is already built into Windows XP, Vista and 7).
Disabling your firewall for a short time, in order to check whether Incredimail is blocked, is unlikely to do any harm to your PC.
However it's far more likely that Incredimail has lost your account settings. Make sure that you know the following:
1. Your username for logging onto your ISP's (or other provider's) servers. That's usually the part before the @ sign in your email address but a few email providers require you to enter your full email address ;
2. Your password for logging on to those servers ;
3 & 4. The POP3 and SMTP server addresses used by your email provider. If you get your email service from your ISP they should be on that company's support pages. Alternatively you can probably find them listed here
http://www.idl.uk.com/email/isp_settings.htm
and/or here
http://www.kitz.co.uk/isp/dns.htm