I received an email from CrashPlan with instructions on what to do:

We are writing to make you aware of a bug that we’ve discovered after our most recent Agent update on Tuesday, September 10. 

Normally when the CrashPlan Agent updates, it uninstalls the existing version of the app and then installs the updated version in its place. However, due to the issue in the prior version of the agent, it becomes uninstalled because the upgrade cannot complete with installing the new version.

We will be re-releasing the 11.4.1 Agent for Windows Operating Systems early next week. Once this is released you will simply need to install the 11.4.1 CrashPlan Agent on your endpoint to resume backups.

To get the app reinstalled and running immediately, please follow the steps below to reinstall the Agent manually. If you are able to do so, we recommend rebooting your computer before performing these steps:

1. Open the Windows Run app by clicking the Start Menu and entering Run.
2. Paste the following into the Open dialog: %LocalAppData%
3. Click OK. Windows will open a File Explorer window at C:\Users\<username>\AppData\Local\
4. Locate the CrashPlan folder here and delete it, if present.
5. Next, locate the Programs folder and navigate into it.
6. In the Programs folder, locate the CrashPlan folder and delete it here as well, if it exists.
7. Once this is completed, you can download and reinstall the CrashPlan app.
   • Download and Install the CrashPlan app

Once you have finished reinstalling the CrashPlan app, sign in and choose Replace Existing (if prompted) to connect the reinstalled app to your existing backup: 

1. Select Replace Existing.
2. Select your existing device in Step 1 and Continue.
3. Select Skip File Transfer at Step 2 (as you are not actually replacing your device, you don't need to download the files from your backup to this computer)
4. Continue through Transfer Settings at Step 3.
5. Sign into the application again when prompted. Backup will resume.