Yes, I can't think of a reason why mIRC would work one day and then stop working the next, unless something on your computer changed. The fact that this issue affects mIRC 6.14, which was released three years ago, and mIRC 6.3, probably means it affects all versions of mIRC.
Normally what happens is that anti-virus and firewall software auto-update their signature files with new signatures that incorrectly treat mIRC as a trojan/virus and prevent it from running.
Which firewall software are you using?
