Sometimes when you try resuming downloads, it says-
Cannot resume downloading the file:It's possible that the address of the file is not valid anymore or your session has expired.Please start the download of the same file from your browser manually, and IDM will try to capture a new address or new session data to resume this download
When you press OK it opens up IE or your default browser to get a new address but still the download does not resume and starts from beginning. And it will get to your nerves when this thing happens when you are downloading a large file and it stops at 90% or above.
Generally what happens is that the address of the file on server changes and it tries to resume from the web page address(the highlighted address below is the changed one. Usually the address is with the same extension name as that of the file you are downloading.)
Press OK and then resume it.