Problem

An excel document consistently opens in ‘offline’ mode.

Cause

If you are using your Office Document Cache as temporary storage  for SharePoint files, the Office Document Cache may be corrupted.

Solution

Clear the Office Document Cache by:

1. Go to Start -> All Programs -> Microsoft Office -> Microsoft Office 2010 Tools
2. Run the ‘Microsoft Office 2010 Upload Center’
3. Under Settings, click on Delete cached files