I was having the same problem numerous times. Yes, rebooting helps, but I am towards stability, so I have figured out how to start the crawler again without rebooting the whole device.
Below are the steps you need to perform:
Ensure you have SSH connection enabled in the WD My Cloud network settings
Connect to My Cloud via SSH and enter below commands to the console
The output should look like this:
WDMyCloud1:~# /etc/init.d/wdmcserverd restart;
[ok] Stopping wdmcserver: wdmcserver.
[....] Starting wdmcserver: wdmcserver9498 (process ID) old priority 0, new priority 0
WDMyCloud1:~# /etc/init.d/wdphotodbmergerd restart;
[ok] Stopping wdphotodbmerger: wdphotodbmerger.
[....] Starting wdphotodbmerger: wdphotodbmerger9649 (process ID) old priority 0, new priority 0
WDMyCloud1:~# /etc/init.d/wddispatcherd restart;
[ok] Stopping wddispatcher: wddispatcher.
[....] Starting wddispatcher: wddispatcher9721 (process ID) old priority 0, new priority 0
Crawler should be started by now and you should start seeing a green flag in your WD dashboard > Content Scann.
During first few minutes it will show that Scan Completed, but 0 files are processed. Don’t worry, soon your old numbers will be back and if you have any changes to your media library - it will start scanning.
It took 5 minutes to restore all the library in my case.
Hope this will help. Cheers