Developing add-in for Office (Word, Excel, Project, etc) you may have to stop programs very often during debug sessions.
When you start again Word/Excel/ecc you may accidentaly disable your add-in.
Re-enable it is very easy, just read here: How to: Re-enable an Application-Level Add-in That Has Been Disabled.
Copy&Paste for Office 2007:
To re-enable an add-in that has been hard disabled by Excel 2007, PowerPoint 2007, or Word 2007
- In the application, click the Microsoft Office Button.
- Click the <ApplicationName> Options button.
- In the categories pane, click Add-ins.
- In the details pane, locate the add-in in the Disabled Application Add-ins list.
The Name column specifies the name of the assembly, and the Location column specifies the full path of the application manifest.
- In the Manage box, click Disabled Items, and then click Go.
- Select the add-in and click Enable.
Have a nice reading.
source: How to: Re-enable an Application-Level Add-in That Has Been Disabled