Surely is you manually start Skype and open its settings you can set to start on boot, hopefully that would reinstate the startup entry. I don’t use Skype so I can’t offer any practical help on the product/settings.
You could also try using the msconfig command open the Startup tab and create a new entry for Skype.
Just select the Quick scan, avastUI > Scan - Quick Scan from the list, now click on the Settings… > Scheduling.