In an effort to ensure Outlook loads quickly, Microsoft monitors and disables slow starting Add-ins. Subsequently, the MailStore Add-in can get disabled and can the Hookshot (formally TdF). If it 'disappears':
In Outlook select File in the right hand pane, select Manage COM Add-ins. You should see MailStore Outcook Add-in. Click on the arrow next to Options and select: Do not monitor this Add-in for the next 30 days. Click Apply and Close
If you still do not see it proceed below:
In Outlook select File > Options > Add-ins. At the bottom of this window, next to Manage: click on the drop-down box and select Disabled Items and then Go. Click to highlight the MailStore Outlook Add-in, select Enable and select Close. Then select Ok.