Too vague. To start, did someone delete the cfide or dfide\administrator directory on the server? Was the virtual directory to the cfide ir cfide/administrator directory removed or disabled? Are you trying to access the site from a non localhost or 127.0.0.1 location with the administrator locked down to localhost? Is CF running?
I am getting this error when trying to get to the admin page.
Error Occurred While Processing Request
File not found: /CFIDE/administrator/index.cfm
Nothing has changed and this CF install has been running fine for over a year.
The only thing it could possibly be, in this case, is magic.
OR... something has changed, and you didn't notice, wasn't made aware of it, or someone did something that they didn't think would impact this (but it does).
That's quite obtuse, I know. But you should approach this problem not from the perspective of "nothing's happened, this is weird" to "something has happened, I need to find out what".
Steve's suggestions are a good place to start.
NB: he speaks of virtual directories for /cfide. You're on a case-sensitive file system, so they need to be /CFIDE. But, yeah... is the dir actually there, and is the mapping to it actually there?