DHCP not working when internet fails

[Jawahar]

Hi,
I have set up DHCP on the Lan interface and it has been working well for the last year or so. There was heavy rainfall and lightning storm a couple of weeks back and the ISP has been having some problems in our area.
The DHCP still works well, but when the internet fails, the DHCP stops issuing ip addresses. When the internet resumes, the DHCP also resumes working.
I could not figure out the connection between the internet connection and DHCP.
Can someone help me find a solution to this?
I understand that without looking into all the configuration, one cannot help. But if you could give me pointers to probable issues, it will be very helpful.

Thanks & regards

D. Jawahar