WP Multisite - Main site is not loading in browser...

Hi,

I have a multisite installation of wordpress with wpmudev plugins installed... which runs past 4 - 5 month without any issue...

Few days back suddenly the main site stopped working. But subdomains created are working perfectly fine. I could able to see those sites and could able to login as admin and play around it. But main site was not loading in screen at all.

If i try by commenting the wp-config multisite lines some error message is coming to the screen(atleast it was loading).

Can anyone help me with this ? We did not install or update any plugins past 2-3 months. Why it happens suddenly ? why only the main site is not working when all the subdomain sites are working fine...!

Pls some one help on this...