I have been getting a temporary blank screen with the message "Sonar (not responding)" since X2 and up to and including Platinum, when loading anything but the smallest projects. Even in cases where the screen may not go blank (the behavior is not entirely consistent), the Sonar screen appears frozen, with no indication of progress. Eventually, the project loads, but it may take a minute or more. I have tried consolidating VST folders, but this has made no difference. Any ideas?

How long have you waited? Before I got my SSD, it took FOREVER....but always opened.

Run a test of your hard drive to be sure there's not delays trying to read data from it.
 
Turns out you can run a disk speed test from device manager (link below)
or from cmd prompt using winsat.
 
https://ttcshelbyville.wo...ows-7s-device-manager/
 
On Win 8 on a VM I couldn't find the gui test, I assume it will be in properties for the disk controller.
 
From cmd prompt admin this worked on win 8:
winsat disk -drive c
Simple fast test to see how things are doing.  Substitute the drive letter for what you want to test.

Not saying its this is your issue for sure but it could also be a sign of a dying hard disk. I had a computer which exhibited all sorts of bizarre sluggishness including slow loads, temporary freezes and blue screens in video drivers. The problem ended up being the hard disk. It simply died one morning.
 
Try doing a scan disk of all drives. If it takes excessively long to complete or reports errors that could be the root of your issues. Data on a certain area of the drive could be very slow to access.

Thanks for all the suggestions; I'll give them a try.
 
Incidentally, my C drive, where all applications reside, is an SSD, and all Sonar project files are on my D drive, which is a rotating drive.