As a last ditch effort, I did the following:
-Checked Rocksmith's files integrity on steam (several times) - didn't work, even though every time it said 1 file is missing and redownloads it;
-Uninstalled and reinstalled Steam - didn't work;
-Uninstalled my antivirus (Kaspersky Total Security) - Didn't work;
-Uninstalled Kaspersky's VPN - Didn't work;
-Checked integrity again - Didn't work;
-Opened as admin - This time it worked!!
I'll try again later on (my fretting fingers are kinda sensitive right now)
Edit: Played for two days and then it stopped working again and I've tried many things and no deal.
Same thing happened to me recently.
It's been a year or two since I last played, and when I came back to it, it just didn't work: Opened up a blank screen and then closed it in about 2 seconds. Nothing I do seems to work. Tried verifying game files on steam, uninstalling and reinstalling, unplugging all USB devices, disabling audio drivers, disabling antivirus, all to no avail.
Please let me know if you find something!