There are many problem due to which this issue has come. Some of then are :
Any entry in you apt.list is not working. You or any application has created a wrong entry.
apt repository are not currently working.
Corrupt in /var/lib/apt/lists/ folder.
So for both of them there are different solution, if you didn't know the exact problem then follow following steps:
Use another mirrors of apt. In ubuntu there is file name /etc/apt/sources.list and folder containing some files in /etc/apt/sources.list.d/. So, from there you can change use any other mirrors of apt. Like you can change your India repository to US repository, etc. by editing these files.
There is another possibility too that you having any other repository link (third party) which is not responding for the moment. So, you can also delete them from the above given files. You can also remove all third party repositories from list
rm -rf /etc/apt/sources.list.d/*
Remove all contents from /var/lib/apt/lists/
rm -rf /var/lib/apt/lists/*
I had this issue on my ubuntu on virtualbox before. I tried all above but that won't help.
When I worked on other backup script and I recognized that my server had incorrect date/time. I have change that and the apt-get update works like a charm.