The program did not disappear. mIRC is set by default to minimize to the tray, however in Windows 7 tray icons are hidden by default. You can expand the hidden tray icons with that tiny little "^" button next to the right hand of the taskbar. I can only assume Microsoft is trying to encourage developers to use the taskbar icon instead of throwing icons into the tray (which current programs love to do).
The fix is to click that little "^" button then click "Customize...". That will allow you to set mIRC to "Show icons and notifications" instead of the "Hide" option. You can also set mIRC not to minimize to the taskbar, which would be the better choice for Windows7.
I really hope they don't plan on keeping this feature, it will likely confuse the hell out of thousands of users since tons of programs have this behaviour. You are not alone in your confusion, I assure you.
