My current Quicken file disappeared, had not backed up for several days, unable to open the backup.

When Quicken closes it automatically backs up your data file to the backup folder on your computer. Your post is a little confusing because you state your current file is not there but then you state you cannot open the backup. If you can't fine your backup file, access this link.
https://www.quicken.com/support/how-find-quicken-data-file-quicken-windows

The fule of thumb is, If your only Quicken file backup is on your computer then your file isn't backed up. I have my Quicken files back up to my computer, a flash drive, an external portable hard drive and to dropbox. It may sound like overkill, but the odds of four backups failing is highly unlikely. If your computer crashes you will lose all of your Quicken data but if you have additional backup resources you can restore your data file. Read this backup information

https://www.quicken.com/support/how-backup-or-restore-your-quicken-data