If your mobile device doesn't seem to be detected and can no longer connect to the desktop (the synchronization is not launched), the problem can be caused by ActiveSync:

Restarting ActiveSync

It can be necessary to restart ActiveSync (Windows XP) or the Windows Mobile Device Center (Windows Vista/Windows 7).

If you want to restart ActiveSync, please check out this page.
If you want to restart the Windows Mobile Device Center, please check out this page.

Sometimes it can also be necessary to soft reset your mobile device (reboot it).

Deleting ActiveSync partnership

If an ActiveSync partnership was established with your mobile device, you may try to delete this partnership:

If you are on XP: in ActiveSync window, Tools/Delete partnership after having disconnected your mobile device
If you are on Vista/7: Windows Mobile Device Center/Mobile Device Settings/End a partnership after having disconnected your mobile device

Caution: Windows Mobile 6.x devices (previous operating system behave differently):

If you setup a new partnership and select Outlook contacts, calendar, tasks or e-mails synchronization, then after the synchronization:
If you delete the whole partnership, it will delete all the items of this Outlook category on the mobile device when you connect it (even before setting up a new partnership) if your Outlook category is not synchronized with another computer within another partnership.
You should see warnings informing you about it when you unselect the Outlook category or delete the partnership.

So on Windows Mobile 6.0, be careful if you have an Outlook category (contacts, calendar, tasks or e-mails) selected for synchronization that you unselect or if you delete your partnership: it can delete all the items in this category on your mobile device.

If you deleted synchronized items on the mobile device, ensure to RESET BirdieSync synchronization of these items in BirdieSync Activation tab to avoid synchronizing these deletions in Thunderbird.