It would be better to know:

1) What the error message is.
2) What version of mIRC you're using.
3) What version of Windows you're using.

Remember, you can check out a number of error messages and their meanings here.

You may also like to check for viruses and try and investigate what program might have caused the shut down. There are quite a few free trojan/virus scanners out there, including SwatIT and The Cleaner. A good virus scanner is Grisoft's AVG or for a free online scan, try Trendmicro.

If you have mIRC 6.12 and Sys Reset script, delete Sys Reset and reboot. You can also run a Search and expand to 'All Forums' and 'All Posts' for best results.

G'luck, hope you get online soon.